We're watching this play out right now with Letters (http://arstechnica.com/apple/news/2010/01/rage-against-the-m...) because apparently Apple Mail.app isn't good enough for power-emailers.
Nobody expects Letters to become the new default OSX Mail client. Rather, it will probably be a successful niche app for power users.
An app like Letters would never see the light of day on the iPhone because it "duplicates system functionality." Mail is good enough, so the logic goes.
Why should we have to have the horrible experience... I don't think it's asking to have our cake and eat it, to be able to play with the toys we've bought?
Should all developers just stick to linux and windows ? - Your walled garden will be a much duller place for it.