Well of course, you aren't really a thief when you play a sneaky bugger in an RPG like that, you're a rogue. Whether you're a thief too depends on whether you use your thieving talents for thievery, versus using them to get all the dungeon loot like you would in literally any RPG, all the way back to D&D. Because how many campaigns have you looting random peasant houses instead of dungeons? :V