Two words: Push Email

Yes, there's a preset icon for both Yahoo and Gmail but iPhone supported push email for Yahoo mail accounts, still does, and still does not have push email from Google.

To get Yahoo push email working before the 3.0 with "push" for everyone, Apple allowed Yahoo's mail a feature only shared on the iPhone by the MobileMe subscription service.

The default search in Safari is also through Google. And to be fair, Maps+Youtube are more significant apps than Weather+Stocks in terms of usage and complexity (should be easy to swap out the Yahoo sources for some other ones).