Heimerdinger might be powerful, but he is certainly not invincible.
But remember; every champion has dark days...that only makes the light that much easier to notice.
Heimerdinger is not made to support, he's made to push. He was born to mid lane. So why break the mold and send him to a place of darkness and uncertainty? Because at some point in a Yordle's life, he needs to be ready; prepared to accept the challenges of the unknown.