For me at least, one of the biggest differences between speakers/subs and headphones lie within the massive disconnect I experience with headphones. Even with the best headphones on the planet, you won't ever get the full body tactile experience you get with a traditional setup. When I'm listening to a symphony and hear the timpani kick in, I want it to feel like I got kicked in the chest like you can feel when you're at a live performance. When you're in front of a full orchestra, you can FEEL the vibrations of all the instruments. You just don't get that with headphones, but you definitely can with a good external setup. Closest by far I've come to replicating the experience was with hearing the magnificent Status Acoustics 8T's. Mmmmmmm.
I do love headphones due to the fact that I have a family (with babies) so sometimes I have to keep my audio to myself, and the audio quality can be absolutely superb with a nice set. Also great for apartment living of course. Still though. In my book, there's just no replacement for large speakers, subs, and gobs of power.