But whoever you are stealing from should become hostile against you. So, what is really to blame is "Busy NPC" bugs.
I'm sorry, but if my character is going to specialize in thieving and he wants to be a proffesional thief, he should gain enough experience by doing his work to master this "art". So he should be able to level up at the same rate as other specializations do. A character which specializes in sniping with small guns levels up by doing this, so why do you take the possibility off from other types of characters... You want to leave this game with combat-capable characters ONLY?
From what I understand, a decent thief needs very high skill (200+ %) to have much success at all. Why should they be relying on a secondary skill just to level up?