When MS decides to change the format of WMA and no longer support the old format, you'll probably have no recourse other than to reencode your entire collection. Since there are any number of open source mp3 implementations, not to mention that the mp3 spec is open (in the old 1980s sense of the word), you're not going to have that problem.

Not to mention the political objective related to Microsoft's kowtowing to the recording industry with their asinine Palladium scheme and the whole concept that I would ever want to restrict the playability of audio files I've made myself.