I've got some decent headphone systems (Audio-Technica W5000/HA5000, Stax O2 Mk I/727II) and I've got some decent speakers (MBL 111E). When it comes right down to it, I prefer the speakers. Why?
1. Cords. I have a tendency to get up and move a lot, which is not conducive to headphone listening. I might be doing something at one end of the room, then have to move to the other end of the room to do something else. Or I might need to go downstairs to get something. With a headphone cord, my movement is limited, and I don't like constantly taking off my headphones and putting them on again. With speakers, I don't have to worry about this.
2. Comfort. Let's face it: with headphones, you are always putting something over or in your ears. No matter how comfortable your headphones are, they will not be as comfortable as wearing nothing. Many headphones require some adjustment to fit my head just right, and even when I perfect the fit, I can't keep them on for more than a couple hours at a time. When I take them off, I get a sense of relief. I don't suffer the same problems after a couple hours with the speakers.
3. Sound quality. Overall, I feel that my speakers sound better than my headphones. The treble is extended, the bass is solid, and everything just sounds perfectly balanced. My headphones don't quite reach the same level as my speakers. Then again, I spent 5 times as much money on the speakers as I did on my Stax system. I think my headphones perform very well for the money, but I'd rather have the speakers.
So if I prefer speakers so much, why do I still have headphones? There are three main reasons:
1. Portability. Obviously, I can't bring my speakers with me on the bus, but my JH13s travel very well.
2. Atmosphere. While I generally prefer the presentation that the speakers provide, there are times when I want to have a more intimate atmosphere. When I have my headphones on, the musical experience is more direct. It forces me to pay attention to the music, and there are times that I seek that out.
3. Domestic harmony. There are times when I want to listen to some music that my girlfriend despises, and the headphones come in very handy in those situations. In addition, the speakers are hooked up to the TV, so when she is watching a TV program that I dislike, I can use my headphones on a second system.