Is the assumption then that the player will never have more than one playthrough?
No, the assumption is that one of those playthroughs will be the first and subsequent ones may or may not occur.
You should balance the game for this first playthrough, although anything that makes subsequent playthroughs 'new' and not significantly less challenging as well as making game more difficult to cluelessly wiki your way through it is a welcome addition.
Oh no. Much worse.
It is under the implicit assumption (implicit since I do not believe anyone supporting it is aware of of its implications) that the game is made by dumb people for dumb people with Attention deficit disorder.
Ok, so I play Fallout for the first time. It's post apoc game inspired by Mad Max. It implies scarcity of resources and strong survival themes, so it seems natural to tag melee as my combat skill (not much ammo to go around), outdorsman (survival, man, you will be walking around scorched, radioactive, post-nuclear wasteland a lot, would be downright stupid to not tag it) and some medical skills (likewise, it's not like there are going to be healing potions or equivalent just lying around in great quantities in harsh, post nuclear environment, right?).
Oops.
Or maybe I play Fallout for the first time and I see it's a retrofuturistic sci-fi game (albeit a post-apoc one), which is reinforced by comics in the loading screens. Naturally I pick rayguns as my primary offensive skill.
Oops.
Attention deficit disorder supposedly means lack of prophetic skills.
If you have swords, bows and staff skills and the retards put precious points in all three then probably something is wrong with their understanding (example).
No, it means something is wrong with your design, because you put in three skills without making them different and circumstantially useful enough to make the versatility of having all three worth the tradeoff in power, making them effectively redundant.
Shit example from a shit poster - how quaint.
If a build is possible it should be viable and potentially fun to play. Now, finding this viability can and should require a lot of sweat, blood and sweet, sweet ragetears.