You're right. But a lot of users buy/download free apps simply because it has a nice icon. Would you buy/download '3420 TOEIC Word Test Pack'[1]? It might be good, but you wouldn't download it because it's ugly and pollutes your shiny phone's home screen.
Well, like most people on HN, I'm an engineer, and I just don't care. I want functionality and stability. You could have the ugliest icon ever and I just don't give a damn because it is irrelevant.
I agree completely, and that's what I do too. But I also understand that potential iOS users are not necessarily engineers and if my goal is to sell more apps to these naive users, I'd better find a good designer.
[1] http://itunes.apple.com/us/app/3420-toeic-word-test-pack/id4...