I used to use Windows Media Centre for my music, video and TV playback.
One of the things that was concerning me about moving to the Apple TV was that it didn't take a TV feed, so we would have to switch between inputs on the TV. To get round that I put EyeTV onto our server so at least it could record shows and add them straight to the iTunes library. It would mean that we could watch TV programs through the Apple TV after a little delay.
As it is, we have found that we don't have a problem with switching. We know that if we are watching TV we switch over to the Sky box and if watching music/video we go to Apple TV. It isn't an inconvenience to change, and it hasn't stopped us from watching or listening to what we want.
In fact, if anything it has widened things up for us. Because Media Centre did all three we never used to move away from it. However, as switching inputs is something that we just do now, we have found that we are more likely to switch over to the PS3 instead sometimes, meaning that that gets a lot more use.
I prefer it how it is now.
I'm obviously not saying that everyone would feel the same, but it's an example of how the result of these things is not always what you would imagine.