I mean really, why do you think people get gear? It's so they can play more parts of the game and have more fun.
Personally, I can't really defend dungeon running that much because I always liked PvP more -- having human opponents to fight was always more fun for me, and the game certainly encouraged that. Some of my friends will still talk about how they were the top of their server or server group, and have folders of screenshots of them beating other prominent PvPers.
I never got very into WoW because by the time it got popular with my friend group in earnest I had switched to Linux and wine only ran WoW at about 20fps. But I had plenty of fun playing it and I barely got into it at all. Saying that the game doesn't encourage it is total bullshit. Finding night elf in Warsong, chasing them down, and watching their health drain from my warlock's curses was some of the most fun I've ever had in a video game. And I got a shiny token for that, too, so there.