Other 'uses' for a profesion Thief' roll that aren't covered in the other skills like 'finding a good target' casing a target , assessing how hard/dangerous a potiential 'job' is, knowing what (and who) to steal (from), and what not (and who not) to. Spotting another thief doing this to you or your party. And to keep the player on his feet just ask for a roll at times...
Thiefy "I'll pick the fat guys pocket"
GM "ok roll picketypocks"
Thiefy "ok thats 28" <smarms>
GM "good you take the Guys wallet, give me a Prof-Thief roll"
Thiefy "err ok, thats, ohh 9...."
Gm <smile>