You had to replace the defaults because they don't work well. Android can't even play a damn wav file without downloading an app. iOS4 has the capability to lock portrait mode. I never once wanted to use my iphone or nexus one as a mass storage device.
The point is, I can replace the defaults, and because of the way actions work, the are proper replacements that integrate with other applications.
It isn't about using the phone as a mass storage device, dummy! It's about being able to e.g. add and remove music, movies, photos etc. to and from the device and a network location, over the air, without ever having to plug into another computer.
Biggest for me is file browsers, being able to copy content onto the device from across the network, not needing to deal with iTunes.
I've replaced the default media browser, video player etc.
And I can use my phone in bed with auto rotate disabled without having to jailbreak.
To me, the primitive nature of iOS stands out. It has a lot of polish, but it is simply less functional, less useful.