I have only briefly considered developing apps for Android (too much trouble; not enough return unless you get extremely lucky), but have developed for other platforms in the past including years of work with PalmOS devices. Anyway, what follows is an educated guess:
In other, similar, operating systems, media services are actually provided directly by the OS and the developer of, say a music player, simply creates a gui which interfaces with the OS media services. For example, all the code behind a "next track" button is nothing more than to invoke the mediaserver's next track function. Volume control? Same thing. List of available tracks sorted by album, by song, by artist? Same thing. In short, an app like a music player is actually rather easy to write because all it is is a pretty interface to underlying services provided directly by the OS. (the same can be said of file system managers such as Astro).
Having said all that, it is probable that Android's Mediaserver is invoked any time any app performs media-related functions. In a rough estimate, if you run PowerAmp, Play Music, Audible book reader, and voice navigation, for one hour each then Mediasever will have been in use for a total of 4 hours.
Again, this is just a guess based on the way other OS's I am familiar with function.