When I was younger I didn't use Yahoo! because I disliked their logo and the use of ! in their name.

Petty? Yes. Did it effect their market share? By at least 1 - yes.

I highly doubt I was the only one to act like that. Design matters. Even if you don't think it does.

E:

For those wondering, I also didn't enjoy listening to Panic! At the Disco. They also eventually dropped the !.