I even own a pair of wireless Bluetooth headphones (nice onces, Sony's) but it's a pain in the ass to keep them charged and deal with pairing/unpairing them to various devices, or connecting to the wrong device when I want to use them with something else, or get them to work with various laptops' shitty Bluetooth drivers, when I can just use a pair of decent wired headphones and stick them into the ubiquitous 1/8" jack and move on with my life.
I personally favor Bluetooth headphones even with all the annoyance because as bad as it is, cords are even worse.
For those who feel differently, I suspect they'll either use the bundled earpods or just leave the lightning adapter attached to their best pair of headphones and move on with their lives. Maybe I'm wrong and everyone except me is rotating five pairs of headphones between six different devices on a regular basis, but it seems like just another case of piling on to resist change.
Not to mention the point made elsewhere in this thread that they bundle the adapter for people who still prefer wired.