In reality, the perfect thief would have no armor whatsoever. They don't need it, as they aren't built for combat like ranger or ninja is. They both have specific skills to attack targets directly, and quickly lay waste to them before they have time to realize what happened. The thief's true purpose is in the name itself: to steal things. If you want combat buffs than don't play as thief. The idea is that they can swoop into a player's base, silently raid them, and even pickpocket the unwary player as they leave. If someone happens to see them they can stay and fight, but it isn't their cup of tea. In that case they can confuse the target, and evade them all together, using skills like blackjack, tumble, kick, and especially sneak. The point of having more armor is completely irrational because, like I stated, combat isn't the thief's skill set.
The idea of buffing pickpocket though could be very useful. I very much liked Thunderjolt's idea, and had a suggestion of my own. Maybe instead of alerting the player with a message, there was a chance (based on the thieves level) that the player would be alerted of the pickpocketing, and could thus turn around and shoo the thief away. This would all occur under a 3-5 second warm-up. As well, the thief has to be BEHIND the player. If at any time in the warm-up, even if the roll was successful, that the victim sees the thief than it is immediately canceled. Also, possibly a pickpocket wait on a player. So if a thief successfully steals something, they can't do it over and over to the same person. Maybe a 3 minutes cooldown on an individual player before they can be pick-pocketed again.