It was also a major temporary power boost, allowing him to defeat both the invading trolloc army and Ishamael. It also killed Aginor. There are also theories that the eye somehow forced Rand, accelerating his natural power development and allowing him to reach maximum strength faster than he would have otherwise. It could also be that pure saidin is less likely to kill you if you don't control it properly, making it easier for Rand to learn to control the power without dying. (3/4 of "wilders" die without proper education or something like that.)

And ofcourse it is even possible that the eye was not used for the purpose it was intended at all, and Rand majorly screwed up by using it up.