While I really like speakers (and I've heard some of the best), I've found that they're really only good for extended sessions when I'm trying to relax, lie on my chair, and just listen to music. But they're useless, imo, when I want to listen to music as a backdrop for doing something else (paperwork, computer, writing, drawing), because speakers are too reliant on sweet spot positioning. Where you sit in relation to the speakers is vital for good sound, and with headphones, I don't have this problem.
Aside from that though, I do think that both speakers and headphones have their sonic advantages and disadvantages. Headphones provide more on-stage intimacy with the artists and musicians, and are less dependant on the room environment than speakers to sound good (actually, not at all dependant). Speakers provide a depth and wideness of soundstage that's similar to listening to live performances. I'm not going to take sides as to which is a better listening medium, but just say they are different and leave it at that.