Even if you don't consider yourself a composer per se, I think it's a good idea to make an attempt to compose occasionally, because it's a good indication of where you're at as a player.
This is kind of a touchy subject for me, because I'm of the opinion that musicians in general are WAY too obsessed with the music of the past. I'll be the first to admit that master composers like Mozart, Beethoven, Chopin, and the like were geniuses, but do we really need to hear any more interpretations of these works? I sort of split my time between classical and jazz, and the same is true of both: there is little room for the modern composer to mark his (or her) mark. If you ask me, nobody will ever play Beethoven better than Ludwig himself, and nobody will ever play Bud Powell better than Bud himself, so why try? Many pianists have the opinion that modern material is a sort of "novelty repertoire", but I say that modern compositions are just as important as works by the classical masters.
A better question would be "Do I ever get to play my own compostions?" In this history-obsessed musical world, just getting a fair chance to play your own work is an effort in its own.