The point, essentially, is that any changes to the base mana/health regeneration system would have drastic effects on PVP. This, combined with the increasing desire to eliminate the necessity of consumables in raids with the 1-pot per fight (so no chain chugging mana pots every fight) means that something has to exist to fill the gap between start to OOM. They can't make the bosses have less HP or take less time to defeat since it still needs to be tuned to have undergeared groups or uncoordinated groups hit an enrage timer.

So, replenishment exists as a decent way to keep casters from running out of mana while in a fight but not in a totally unbalancing way.

Also, as a Hunter I haven't seen a single rogue out DPS me in this expansion so far. Ever. If you want the best of everything a Survival Hunter is going to get you a lot of great utility with misdirects and replenishment as well as top tier DPS. Same goes for a Ret Paladin which I've seen top the DPS charts often enough too. It's not that you're sacrificing anything at all bringing these guys, you're really sacrificing more by NOT bringing them if you want to bring a Rogue instead. Unfortunately, Rogues pretty much are crap right now for anything except just standing around and stabbing things now that Sap is all but useless in most instances.