I can quickly turn around aim and headshot someone, so therefor I use that fast hand and eye coordication too quickly navigate and click to whatever button on a MMO skill bar I need too.
The reason most players keybind is because they find clicking clumbsy and slow. Well as a FPS gamer Im use to Precision and Speed so for me Putting my mouse cursor over a skill on my skill bar is effortless and faster then keybindings for me.
ive tested this theory when I was in Exodus Guild in WoW which was one of the top rated Guilds in North America back when Kaywarrior was running it...
We used Recount to test Me as a Clicker Disc priest Versus a Keybinder Disc Priest. By the end of 5 Boss fights I matched The Keybinder in not only HPS but Decursing aswell. Now do bare inmind as I said I do use a Razer Naga so I dont completly use just mouse clicking, but about 80-90% of what I do is Clicking.
Hope this clerifies some of your questions.