Originally Posted by
marukale
Clicking advantages:
If the mouse curser is over the button then clicking is faster than key press (compare mouse button sensitivity to keyboard sensitivity)
Higher density of buttons around focal point, optimal distribution of click-able buttons around the cursor (ie focal point) is 8 plus the one on the focal point assuming standard three button mouse used that's 28 abilities that can be clicked without watching the cursor (muscle memory works just as well for clicking with a mouse as it does moving to a key), keyboard optimal distribution of buttons is roughly 15 but depends on hand-span and if you use 4 fingers or 5 (this does not include modifiers as that is a separate point)
Higher CPM (clicks per minute) due to higher mouse button sensitivity without impact to joints. Can't think of a reason why in an MMORPG your CPM is important.
Focal point reset easier without looking, ie if you move your hand from a keyboard it's easy to put it back down in the wrong place, with your mouse a ui can reset around the cursor wherever it is.
Disadvantages
Mouse turning is more complicated (not impossible as some people seem to think)
modifiers requires two hands (or extra mouse buttons)
Less abilities at focal point, assuming 3 button mouse you have 3 abilities that don't require moving a cursor, with keyboard you typically have 4 or 5 that don't require moving fingers to different keys.
However other things are a much bigger factor, ie does the player know how to strafe, do they keyboard turn, do they use macros etc. Do they have a ui that supports their chosen method, the standard ui is really not suitable for clicking but can be used fairly well for keybinds but still isn't optimal.
The biggest difference between successful players in both PvE and PvP to unsuccessful is the ability to predict the next ability they will use and being ready to press it before it is ready, if they manage that 100% of the time then clicking is better than keybinding but managing 100% is pretty much impossible especially in PvP.