Rakan is fun, Xerath is about damage. Unless youre a god on Xerath with your EW, you're not going to be able to peel.

Play rakan, but you're going to have to learn how to time W. W is not about just getting a knock up. Rakan's W gets canceled if someone sneezes hard enough. His ult doesn't have that long if a duration and you're usually not building tenacity on him, so you're going to have to time yourself.

If you want to play a damage mage support, you might as well go Annie, Hextech (or whatever its called now) and flash engage. Annie's threatening enough as a support with just her stun up and she's one of the few champions in the game with a point and click stun.