The Thief's 10 point bonus to Luck makes it much easier for the player to boost all of the attributes to 100 in the long run.
(Summary and picture from Official Elder Scrolls Site)
The Thief provides the biggest attribute boost in the game. No other skill modifies Luck so boosting your Luck is a valuable asset for characters who like that skill. Luck affects the Marksman skill, and Mastery perks as well as pickpocketing and sneaking. Choose this birthsign along with Luck as a favorite attribute, and you can easily get your luck up to 100. The Thief is also great for sneaky character builds.