Good PvE players don't click (unless you are a healer and clicking the ops frame). 6 button rotation can be done on 1 - 6 with aoe/circumstancial moves on things like Q, E, T, F, G, etc and it increases your speed because you press the button as soon as it's off cd, having to move the mouse over and clicking will cost you 0.25-0.5 seconds, now if you are on an 8 minute boss fight imagine how much of that time is wasted finding a button and clicking it? You may be fast at clicking but you still have to move the cursor over the spell, etc if the fight is short it won't make much difference but on a longer fight where dps is tight (Firebrand and Stormcaller Nightmare for instance) you will cost yourself a good few hundred dps, if every dps does that you will be about 800 dps down so you will cause a wipe.