Since Lacrymas tagged me I ended up reading the discussion on this page. Can't speak to what the game is like now since I haven't played it since shortly after the 2015 release.
A few points:
AI Targeting
In my experience, most AI will first and foremost attack the closest enemy initially sighted. I do recall Baldur's Gate and Icewind Dale: Heart of Winter having some cases of AI switching targets based on a preference. HoW AI from memory appeared to be a bit smarter than BG1/BG2 default AI. Since the games were made for slow computers and to not be too difficult (apparently BG1 was way too hard initially so they had to make the AI dumber), it takes a while for the default AI to change targets, in those games I recall often the Mage being the character that the AI preferred to target because they often had the worst AC. In Baldur's Gate and Icewind Dale in particular it's not very difficult to set up the fight relatively optimally before the engagement because units don't move very fast, so if you use pause correctly and you see that Ogre Berserker going for Imoen you can just move her away, move a character with Plate Mail in front, and the Ogre Berserker will retarget the closer character. The Ogre Berserker has a large selection circle so it's very easy to block/control those types of monsters in BG and grid-based pathfinding in tile-based 2D games have a very consistent feeling to the movement and collision of units that makes it very easy to get the hang of.
I recall the initial Pillars release was programmed kind of similar except the AI would choose a new target pretty quickly. I played on Hard and I recall Aloth often being targeted no matter what because of his low defenses. It got to the point where I was playing with Aloth standing far at the back out of the initial engagement to avoid the aggro, sending other characters in first and having him come in at the end of the Alpha Strike, and even then enemies would often retarget to him, and disengage the melee characters. It often meant low health enemies would get whalloped by the engagement system but it was a bit annoying for tanky or hard to hit enemies. The way the AI worked at release meant that it was optimal to do weird stuff like heavily armor a targeted character like Aloth, and have a character like Eder in very light armor, or deliberately manipulate a character's defense so that they would be targeted which was relatively easy to do with a couple of the stats by juggling equipment.
Can't comment on what it's like now, but judging from this post, it's still the same as release.
And the PoE enemy AI literally seeks out enemies with lower defenses.
Engagement system and "Fake" move past
Baldur's Gate was a lucky accident of an RPG being made with an RTS engine, so the gameplay inherited a lot of the good things about RTS style combat and gameplay. In Baldur's Gate 1 in particular, ranged units are overpowered - due to a combination of factors including the low level D&D system design, the slowish movement speed of units, targeting and AI of characters and the level design of the game with lots of big open maps (wilderness areas in particular). I do not recall this being the case in the other Infinity Engine games, it's something specific to Baldur's Gate 1. In BG2 the higher level of characters meant that ranged weapons weren't *as* deadly and characters had more means of dealing with a ranged character and levels were smaller and more close quarters, and in Icewind Dale the level and encounter design was quite different - much more close quarters fighting a lot of the time, among other things.
Looking at BG1 specifically, ranged was very strong for a variety of reasons.
* First and foremost because of the relationship between damage and hit points. Arrows in particular dealt 1D6 damage. The Composite Long Bow which can be found from very early on adds extra damage. Characters can have less than 10 HP at Level 1 and if you play with Core Rules and get a bit unlucky with the HP rolls, can be killed after being hit by a few arrows. Enemies tend to have average HP and thus particularly vulnerable to arrows.
* Secondly because of the relationship between THAC0 and AC values - Bow users are either Warriors or Thieves, or monster equivalent, and tend to have a higher than average Dexterity and Bow Proficiency so they generally have the highest chance to land a hit than most other types of characters or monsters, particularly in the early game. The regular Long Bow and regular Composite Long Bow also improve THAC0. Enemies tend to have average-ish AC for the most part and are also fairly easy to hit with Arrows.
* Third, Bow users tend to have higher APR than melee characters. Melee characters will catch up, but a Bow user will have at least 2 APR to a Melee character's 1 or 3/2 in the early game, so they attack faster (with a higher chance to hit, and remove a good percentage of HP per shot).
* Fourth, Range advantage - Bow users will tend to fire one, if not two arrows before a melee character reaches them in combat, let alone has a chance to attack due to a combination of character movement speed and how the combat loop works - particularly in BG1 with the 'fake attacks', a melee character may have to make several fake attack animations before a real attack occurs, and when it does they don't have the best chance to hit.
* Fifth, kiting. This is the point that everyone remembers and complains about. The player character can simply run away from a melee enemy with their ranged character and reinstate the top four listed advantages. There's also the fact that enemies can lose morale and run away, and it's actually pretty annoying to chase down fleeing enemies because it's hard to even catch them to hit them. I believe the engagement system was a partial response to the player's sentiment from Baldur's Gate 1 specifically - even though the design of future games on the engine had reduced the prevalence of the issue. I also think that with consecutive titles player skill increased so there was likely less need to 'resort' to kiting to win because of better understanding and familiarity of the systems and gameplay.
The ideological battle over Attacks of Opportunity in real-time games and Pillars of Eternity's engagement system is very simple. It all stems from the perspective on how RPG combat should be represented. RPGs started with tabletop as turn-based systems. Realtime gameplay began with video games. Turn-based games have strict rules that control the sequence of events in the game and limit the amount of actions that can be made 'per turn'. Actions in turn-based RPG games are a reflection of a character skill's and the player's 'skill' is limited to the decision making that goes behind the choice of the action. Realtime gameplay opens up another avenue of player skill - the amount of decisions and actions they can perform in quick succession. Realtime with pause is simply realtime gameplay the ability to pause to have more time to think and issue commands. There is more opportunity to use 'player skill' in realtime games - the ability to think quickly and perform actions quickly and the precision with which those actions are performed. "Unit control" is a player skill, not a character skill and you can use your skill of unit control as an advantage in games that allow it. The people that like the engagement system and AoOs in realtime are the people of the belief that character actions should be a result of character skill, not player skill. I am an RTS player, before Baldur's Gate I played Warcraft 2 and Age of Empires 2 (which I still play now) and in those games, you run away and kite with ranged units, you micromanage low health units to the back to be healed and it's up to the opponent to actively stop you. The opposite school of thought to that is often held by people who prefer tabletop and turn-based games, MMORPG and RPG players - that you have to 'pay extra' to move. There seems to be overlap between the character skill and pay-to-move people. RPG designers also tend to be these people and unfortunately that and a strong overreaction to the memories of BG1 ranged kiting is how we ended up here today.
