Well, I think some games are more user-friendly when it comes to using keyboard to play.
I mean the default WoW keybinds are pretty bad: it's impossible to use 1234567890-= with one hand.
So one is forced to do their own keybings, and some people are too lazy to do that.
I used to click, but then slowly changed to keybinding and it's much better.
As the people above me said clicking forces you to look at your spell bar instead of what's happeing on the screen.
My binds are quite unusual:
I use W to autorun and FG to strafe, because I find it really hard to strafe with AD and use spell at the same time.
My spells binds are all the same across characters, not 'character specific', and I tend to use same place for similar spells (my kick, brain freeze, counterspell and pummel are on shift-q and my disarms are on shift-s).
I use my right vertical bars for proffesions, quests and comsumables (flasks and food).
I sometimes do changes in my binds, as I don't think they are perfect.
First I would keybind spells and then click them anyway, but as time passed....
I think even people who say they click don't really click all their spells, they just use 12345 binds and click the rest, because they don't reach other keys.