You'll learn something, additional career opportunities (maybe) and improved prestige among the developer community (guaranteed) come to mind.
It never hurts to have more than one language in your quiver. If you're looking for work and are having a hard time finding a PHP gig that suits your fancy, having some other language dialed to the point you can code in it professionally increases your odds of finding (and keeping) a job.
Of course, I make a good living coding PHP for high end websites, and have been for years now. I just don't talk a whole lot about the language that pays the bills since invariably mentioning PHP is your primary language leads to a torrent of hot air from other developers who use "cooler" languages. Personally, I think it's all bullshit and if folks want to play "cool kid" with their language of choice, that's fine, but don't expect me to play along.
Fun fact: the highest paid programmer I've met to date coded exclusively in COBOL. Think monster 1099 rates consulting on legacy mainframe code overhauls.
