Yeah, that's pretty much the only reason I'm against it, because balance-wise it might makes sense instead.
I strongly disagree with the suggestion of altering thaco progression.
It's one of the core rules of AD&D.
Ehm...STR18/00 means +3 bonus to hit, and you're forgetting proficiencies points, which means at least another +2. Thus, a commonly equipped warrior hits a commonly equipped opponent 90% of times. BUT, the problem is that player's warriors are exceptionally equipped (+4 weapons isn't the only difference, they can get tons of items to increase STR/thac0) against commonly equipped opponents (let's face it, most opponents within BG2 have a pretty bad equipment, and more often than not even even boss-like opponents have great equipment). Long story short, player controlled warriors hit 100% of times within mid-late game unless the targets are magically buffed/protected.
A fighter with the same equip has :
Base thaco @lvl 21 : 0
Longsword +3 : -3
Strength 18/00 : -2
That's -5. They hit 75% of the time.
That's already planned, though it won't be a complete nerf. IH will grant +2 apr instead of 2x, which means warriors won't be able to "exploit" it gaining +3-5 apr (dual wielders won't be advantaged anymore), but all non-warriors will benefit from it twice as much (they only get +1 apr from it in vanilla).
If you want to nerf fighters, nerf improved haste instead, tht grants them 2-4 attacks per round for 1-3mn .