Taltamir, you recommend a single class thief for BG1. Why is that? Just curious is all. I figured a single class thief wouldn't be able to hit anything much less backstab anything. I maybe wrong of course.
I had a brain error and somehow forgot what the OP asked about halfway through reading the answers people were giving him. I also really should have phrased it better. I would say my recommendation is actually for each and every character be a 2 class multi-class. :)
When I said straight classes are better I was thinking casters and fighter types, who benefit more from specializing early on (higher spell levels, or better fighter stats). I just didn't think about thieves who are to me Imoen, sitting in the back plinking with arrows while the combatants actually kill things... the 6th party member assigned for scouting, traps, locks, pickpocket, etc rather than being a real combatant. Silly considering this is titled "theif/fighter question".
So, that being said let me do a reevaluation specifically for a thief/fighter v thief...
Well, thieves do need to those extra skill points with the level cap being so low in BG1
BG1 XP cap is 89,000, while in the Tales of the Sword Coast expansion, the limit is raised to 161,000
@89000XP your max level is 8 for a straight thief (thac0 17). For multiclass its fighter 5/thief 6 (thac0 16)
@161000XP your max level is 10 for a straight theif (thac0 16) and 7 fighter/8 thief for multiclass (thac0 14, +0.5 melee attacks per round)
Multiclass fighter/thief can also specialize in a weapon for -1 thac0, +2 damage, +0.5 melee attacks/round.
I am assuming your thief is using a shortbow for safety though (let the tanks tank :P), so those attacks per round bonuses disappear.
In both you are sacrificing 50 points of your much needed and fairly scarce skill points for a small but meaningful damage advantage which you don't really need (thanks to the rest of the party being properly min/maxed). Also an HP boost. Also, this doesn't take into account you potentially using stealth in combat.
Eventually though your thief gets all the skill points s/he needs at which point you want to dual class out of it. But this is not going to happen in BG1. People love dual classing imoen in BG2 into mage.