Problem with the "true" grandmastery mod is that it's a very big boost to fighters, and leaves other classes like paladins comparatively much weaker.
Except that it doesn't, it (grandmastery mod) just makes them comparable to each other (as in you have an actual reason to choose a fighter other than dual-class) with fighter still arguably being the weakest class.
-Paladins have turn undead (very underrated ability, can explode undead at higher levels, liches become a joke), can cast low level cleric spells (Armor of Faith being the strongest as after a while it gives paladin a damage reduction comparable to Barbarian) and eventually gain access to the paladin class specific weapon which just happens to be one of the best in the game. Of course for example Inquisitor paladin kit loses on all those abilities but gains dispel magic cast at speed factor of 1 that acts as spell twice his actual level which basically makes inquisitor the best "fighter" type single class in the game (and a nightmare for mages), period.
-Rangers have access to Armor of Faith as well, stealth (scouting is important in BG), favoured enemy and gain 2 proficiency points to two-weapon fighting. When it comes to kits while Beast Master is mostly crap, Stalker gives you moderate backstab and some mage spells with virtually no drawbacks and Archer is the best ranged class.
-Barbarian has Rage ability (+4 to STR & CON and immunity to charm, hold, maze, level drain for 5 rounds ), more hitpoints than any other fighter type class, has damage reduction (that caps at 20%), is immune to backstab (not as useful as it should be but still comes in handy on few occasions) and moves faster.
It also makes it make no sense not to dual over from a 5-point class (don't rangers get 5 points too?) to say a cleric if you want a melee oriented one. It would obviously make sense to do so even without the buff, but it goes from sensible to mandatory.
Again, it depends, some players like having access to multiple HLA polls and dislike the babysit/useless period when it comes to dual classes.
Rangers can only get weapon specialization (two stars), same as Paladin and Barbarian.
As everyone who isn't playing the game for the first time has likely planned out what specific weapon they'll use for their fighter it wouldn't make sense to not go 5-points in that weapon skill anyway.
No, in vanilla BG2-TOB it makes no sense to go 5 points whatsoever, getting piddly +2 to damage and -1 to speed factor or something just isn't worth the investment, especially considering that unlike shitty modern RPGs in BG2 weapons differentiated in more than just DPS and many of them had abilities that were useful for different situations (I won't give specific examples as I don't want to spoil the game for the OP) so it pays off far more to be specialized in multiple weapon types than having grandmastery in one.