The original drakes were bound to their creators, and only lived a couple hundred years. While they made for good spies and 'liasons', they weren't like Immortal Elves, where the only way you could tell they were different was by having them kick your ass in a mage duel. They were also infertile, however, which posed its own set of problems. The drakes we see today are the second version, created by Doll Maker, Mountainshadow, and friends. They were less powerful than 'true' drakes, but could breed, giving the possibility of a replenishing supply of servants.
As for why drakes? Because the dragons still required servants that didn't die as quickly as some of the Young Races, and the Elves were NOT going to sign up.