Physiology of sound
There's also an issue of listening fatigue. Headphones make sounds appear to come from inside of your head. This isn't natural, and your brain knows it. A few deathmatches too much, and you'll be feeling nauseated and dizzy before you know it. All that stale delivery pizza can't help, either!
Well, maybe it's time to look towards the future. Listening fatigue is definitely something that's got to be fixed. Take a look at the head related transfer function technology used in today's 3D sound cards. Their "binaural" technologies are made to make sounds appear to be coming from around you (instead of from within you), and done right, they completely eliminate listening fatigue.
Speakin' of the future, it's apparent to me that 3D positional sound is gonna be the "next big thing". I've read your MX300 vs. SBLive review, and while it's clear that you think 3D audio hasn't yet reached its prime, I did notice one important fact. The main choice people are going to have to make is between 2 and 4 speakers, or rather, 4 speakers vs. headphones. There are numerous studies (NASA and otherwise) which point towards headphones as the best way to convey 3D sound. Not only that, but who has the money or space for a proper four-speaker setup? Furthermore, consider the fact that there's no way any kind of speaker setup is going to be feasible at a loud LAN party or competition. Also, virtualized Dolby Digital for DVD is the perfect solution for those without budget or space - in this regard, headphones can help.