It’s no secret that practically half of the community uses Revolution. There’s not much wrong with it, but at high-end PvM it can be hit or miss, for several reasons.
Revolution cannot use abilities strategically.
Revolution will always use an ability at the beginning of the sequence as soon as it comes off cooldown. This can lead to wasted potential like Needle Strike to Fragmentation Shot instead of Needle Strike to Snipe, or using a same-typed bleed on an enemy already under that bleed, wasting the full duration of it. By far this is the biggest reason why going full manual is better DPS over using Revolution.
Revolution limits movement.
Revolution only works if you’re standing still, leaving you hanging at the mercy of certain boss mechanics. If you have to move, you can’t use abilities unless you actively override Revolution with keybinds. At places like RotS, this can be a deal-breaker due to stopping and going.
Revolution cannot cancel abilities quickly.
Fury, Snipe, Concentrated Blast, Assault, Destroy, Asphyxiate, Rapid Fire, you name it. All last longer under Revolution if not touching the keyboard. To be fair, this can also be a problem for full manual, but Revolution isn’t smart enough to use all of those basic channeled abilities one after another for more DPS (like Fury, Snipe, and Concentrated Blast), whereas someone in full manual is more aware of the situation and what to do.
On the other hand, there can be good uses for Revolution.
Revolution saves you from panicking.
Revolution lets you keep on attacking while you panic eat from a nasty special attack from a boss. Good examples like this are at Araxxi as a newcomer. Using abilities manually and prayer switching can be nerve-wracking, so using Revolution and just focusing on Anticipation/Freedom and prayer switches is good enough on its own.
Revolution saves you from lag.
If having bad ping or FPS, Revolution lets you keep on firing abilities while the game is chugging. This pretty much goes without saying.
Revolution allows you to learn the boss easier.
As your focus is not on what abilities you’re using and more on the boss and its actions and patterns, you can learn easier. This is all subjective to person to person, but it’s generally easier to look at the boss patterns without worrying about your own ability rotation.