Thousands? Most that don't a) are games b) have goofy design.

You have to be very careful with redesigning native controls (see drop-down lists on the web), most of the time it's just an excuse for making the app not look "plain", instead of doing good design.

Thousands out of over half a million. It's very rare to see the default rounded rect button used anywhere anymore. It's even rare to see apple use it in their own apps.