This is an apples and oranges topic. I imagine that most of us would agree that there are certain situations that dictate headphones over speakers or vice versa, i.e., not wishing to disturb others (headphones) and listening to music in a group (speakers) for example. The only fair comparison is being alone in your favorite music listening space with your best speaker rig in front of you and your headphones in your lap hooked up to your best headphone rig. Which one do your listen to?
For me, the answer is "it depends." First, what kind of music do I intend to listen to. Some music favors speakers (classical symphonic music for example or basic 50s, & 60s rock and roll). Music that needs a large sound stage definitely favors speakers, and this includes most dance music. Other music favors headphones (classical chamber music or most solo piano music for example). Music that requires only a smaller sound stage. Other music, such as jazz or Pink Floyd & Dire Straits, can sound pretty impressive through good headphones or good speakers.
I like choices, that is why I own several sets of headphones as well as having more than one speaker set up. A $350 set of headphones, listened to through a decent headphone amplifier, is never going to sound as good as a $10,000 (or for that matter $3000) speaker rig. It is mainly a matter of sound staging and sound range. Likewise, my AKG 81DJs do not sound as good as my K701s. But, you know, I listen to both. They sound different and I like variety. Right now I am listening to a pair of used DT911s and they sound really great. Across the room are my K701s and my HD580s. Am I wasting my time listening to the 911s, I don't think so. I like the variety.
Your favorite cd (Dark Side of the Moon for example) sounds different through my K701s and my OBH headphone amp, than it does through my Mission speakers and my Dalquist subwoofer, both with the same high end source. The headphones really close in the sound stage and you hear some details much more intensely. I honestly do not know which one is better (probably the speakers) but that is not the point. They sound different, and I really like the difference. It just depends what mood I am in. Often I listen to the same cd through speakers and then headphones, back to back. And then listen to a third time through different headphones. It is like listening to three different music experiences.
I would not trade any of my speakers for headphones or any of my headphones for speakers. Enough ranting. This discussion is similar to discussing politics or religion, most of us will not agree but we will enjoy ourselves immensely, disagreeing.