Please don't uselessly pick at people's arguments. This entire discussion is subjective.
He brings up a solid point, especially on PvP servers. In Vanilla going afk was always dangerous, do you risk stepping off the road and behind some cover in hopes no one will come by? Would it be better to ride back to town for safety? Since the advent of flying the clear and obvious choice has been to mount up, fly way up into the air when you are waiting for something or going afk.
This is a very clear difference between flying and no flying. And it does create a more social environment. Take the timeless isle for example--specifically, Huolon's spawn point at the end of the first bridge. It is a location that almost always has a group of people hanging around waiting for Huo's spawn, it is also frequently the location of world pvp. There is a sense of the existence of a server's community there that doesn't exist with flying, because if flying were enabled, people wouldn't be gathered in that single spot near the wall where you are safe from mob spawns, they would be spread out all over at dozens of different heights, or they'd just wait by the celestial court because on a 310 mount they could fly directly to the spawn in less than five seconds.
If you really want "facts" then I'll point out that Shadowpunkz is entirely correct in his assessment of flying making the world feel empty.
Just by virtue of limiting everyone to a single location on the vertical axis, no-flying DOES pretty much guarantee that you see and are in close proximity to more people than when flying is enabled and people can move around at varying heights.