yeah, using a pair of good headphones alongside an accurate monitor set up can have advantages but mastering soley with headphones is pretty much a no no.

like swannie said the way the stereo image and certain frequencies will behave in headphones will not be true to how the sound will move across a room, and also your seperating both l and r completly, speakers/monitors in a real room are allowed space between your ear and also there is an interaction between both speakers so each ear is hearing sound from both speakers as aposed to soley left/right.

not to mention they will wear your ears down considerably faster then a good pair of monitors performing in a space, or at least that has been my experience when using headphones to work on things.