It's not "required", but it seems that everyone but a select few have them. What better place to get your music played than in a university? Other parts of society really don't care about new music…

Technically, one doesn't need any sort of education or even know how to play an instrument to compose…but if you want to write something that sounds good AND people enjoy playing (it's funny how many composers don't give a crap about that), I find an education helps improve that. And it's a technicality, but it's actually a DMA, not a PhD since it's an artistic/performance based field and instead of a paper, you write a composition.

I already know where I'd want to go…unless it looks bad to go to the same school for your DMA as your masters? I love the professors, the city, the program, and the fact that I can keep playing my instrument like when I used to be a performance major. It seems silly to go somewhere else at this point, although there's the argument of having variety.