Hey,
To get a true representation of your music, you should monitor with speakers...the biggest reason I've heard about is because your ears hear audio from both speakers, which is how the music will be listened to in a car, home stereo, etc.; with headphones, you are getting discreet channels sent to the left and right ears and are therefore missing out on some of the music.
Headphone monitoring is not bad, though, because in my opinion you can really hear some of the details that you might miss out on with speakers. That, and you don't piss the neighbors off!
Take care,
Nick