Do you listen to a lot of music or watch video? I listened to a little music lately on my phone, and the usage for Media Server is about #5 on my list... I don't listen to music on my Tab, and media server doesn't even show up.
I don't know the Android architecture, but I suspect that any process needing to play music or video may direct requests to the media server, which would have a generic interface to the apps, and that would deal with whatever the underlying hardware is for each specific device. This would keep apps from having to code for a variety of hardware.