Unless you are the kind of person who shot a hole in your TV's picture tube back in '96 and haven't seen fit to replace it, you've probably seen the NCIS commercial where Abby claims that nothing is more addictive than pistachios. Well she's wrong. Headphones are more addictive.
Currently I have eight pairs of full-size headphones I use regularly. They are, in no particular order, Stax Pro-Lambda Nova, Audio-Technica W3000-ANV, Grado RS-1, Sennheiser HD-600, Sennheiser HD-428, HiFiMan HE-300, Grado SR-80, and Grado SR-225.
I have even more in-ear models, including a two pairs of Shure SE-500PTs, as well as a pair of Shure SE-215, Etymotic P-4, Etymotic E-6i, Ultimate Ears In Ear Capital Reference Monitors, B&W P7, Munito 9mm SITi, Ultimate Ears E200, Sennheiser CX500, and at least a ½ dozen pairs of Apple ear-buds sitting in iPod boxes. Along with the in-ears come several bags of ear-tips of all varieties - triple and double flanges, as well as foam and silicon in a multiplicity of sizes and shapes. Most will never be used, but every once in a while I find the perfect eartip for a new in-ear buried at the bottom of one of the bags.
So, Steven, why so freakin' many headphones? Because, they all sound different. And sometimes your listening environment determines which earphone is most appropriate. I don't use full-size open-backed headphones at the gym, and I don't use in-ears when I'm recording (except on-location where I often monitor with the Ultimate Ears In Ear Reference Monitors). On airplanes I use either the Etymotic P-4s or Shure SE-500PTs, both with a triple flanged tip for maximum isolation. At home on my desktop the two over-ear headphones getting the most ear-time are the Audio-Technica W3000-ANV and Stax Lambda Pros.
Another reason for owning so many earphones is that they take up so little space. My entire collection could fit easily onto one regular-sized shelf. In comparison, owning more than a couple of pairs of floorstanding full-range speakers can be a huge logistics issue. At 570 lbs each, my Dunlavy SC-VI speakers haven't relinquished their place in my listening room since 2001. In my other listening room, figuring out where I'm going to stash my AV123/Skiing Ninja X-Statics and Genesis 6.1 speaker systems while I review something new is always a major domestic issue.
One last, and if I may say so myself, somewhat macabre reason for so many headphones is simple - if I'm ever forced into a retirement or hospice situation I'm going to need a good, no a GREAT pair of headphones, since they will probably be the last stereo gear I'll hear ever music through, and I want to go out smiling...