Er, okay. So stick to one class skills. Like someone else said, you don't need to join the thieves' guild (it lowers your reputation, too), and experience is abundant enough that by the time you get to the Wastelands, you're so incredibly powerful that you don't really need to be more than 40th level to beat the game.
I understand the concept of more distinctions between classes, but I think that if Larian expanded upon the "special abilities" of each classes and included more than one, people would immediately see more differences between the classes. Also, there are some subtle differences as well, such as survivors gain more defense than offense for each dexterity point, warriors gain more offense than defense, and mages gain a smaller but more equal amount of both.
Also, if you insist on roleplaying, keep in mind that you're trying to join the thieves' guild, which itself implies that you have to be skilled in thievery. I mean, you didn't think that your entrance exam to the thieves' guild was casting meterostrike or killing orcs, did ya? So, Larian did the right thing by having pickpocket as an requirement if you want to join the guild.
You have some good points:
1. There are ways that Larian could further distinguish the classes, thereby strengthning them. Multiple class special abilities is a good one.
2. You make a good point about role playing. No, I didn't have to do that quest. Yeah, I am joining the thieves guild, so I should have theif skills right?
My point:
I here the word "Josephina" and "imps" thrown around this forum all the time in discussing how hard they are. Rose, have you ever played a game in which you developed a character, had fun developing his skills, only to find that by the end of the game, your character was stuck because he or she couldn't kill a boss or finish a certain quest because he or she had the wrong skills?
I have had that happen. Several times. Frankly, it sucks. I'd like a little peace of mind in knowing that my charatcer will potentially be able to finish the game. Solution: Make it so that I can invest my points in one set of skills (yes! one set! No need for my warrior to lock pick, throw meteors, summon, etc.) and still have the potential to beat the game.
Thats all I'm getting at. If you want to make a warrior/theif/wizard/pizza delivery guy/window washer as your character, do it! Have fun! But still let me know that my pure warrior, wizard, or thief won't have to become anything else just to survive or kill a potentially difficult boss later in the game.
Now tell me, If we can both do it our way, where am I wrong? You can branch out, I can stick to my warrior, we can both succeed. Sounds good to me...