That depends on the encounter. Some encounters melee just stack and DPS while ranged have to deal with more movement and decursing.
While cleaving packs you're going to be using cleave, whirlwind and/or sweeping strikes. Cleave uses a lot more rage than heroic strike so there's less opportunity to pool too much between cleaves. As a percentage of your total damage done, missing a cleave every now and then isn't going to make or break you.
Even frost mage with a 1 button rotation is impacted by not being able to manage movement efficiently. If your cast bars aren't perfectly synced and one mage moves while frostbolt was 75% cast, then repositions and begins casting again, that's 3-5 seconds you've done zero DPS. As I've said, you won't be able to execute any classes rotation flawlessly regardless how simple because you won't ever have the same level of control as a single player.
The nice thing about vanilla is that doesn't really matter. None of the raids are that difficult that you're struggling against enrage timers as long as everybody is at least competent.
If you're that concerned about micromanaging hs/cleave you could always make a bar swap macro to manually manage when each of your characters dumps rage. I just think the DPS increase will be minimal versus the extra effort.
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