Apple has rejected another application from the App Store this time because:
… Your application duplicates the functionality of the built-in iPhone application Mail without providing sufficient differentiation or added functionality, which will lead to user confusion. …
If Apple was enforcing the same rule for Mac OS X, I could not use Thunderbird which I find more usable and responsive than Apple Mail.
The iPhone could only gain from better applications and better competition (if only to push Apple to add landscape mode to MobileMail...).
Less crappy widgets, more usable applications and let the users decide.