Just decided to start a "pure" thief. Got an Argonian, made her as short as possible, coloured her lavender (because why not). She gets zero points in combat, defence, magic, any of that. Her skill points are being given up between Sneak, Speech, Lockpicking and Pickpocket and those are the only perks she's getting at all.
She's turning out to be considerably better in a fight than my two-hander Orc was. :D

Seriously, the perks at level 100 Sneak and level 100 Pickpocketing combine so well! Duck down mid-fight and pick their pocket while obscured by shadow, and suddenly they're naked and unarmed. And my total lack of combat perks is no longer an issue!

If there is something in this game more hilariously entertaining than being an Offensive Pickpocket I've yet to encounter it.