Requesting a Rebirth on a fallen player during a raid boss is generally the job of the raid leader. Waiting for the order isn't laziness; battle-rezzing someone and being on cooldown when it's needed for a critical player (tank, healer, etc) can be the difference between a wipe and a kill on new progression content. Shouldn't be a big deal on farm content and probably isn't relevant in PUGs, but it may become a habit to wait for some people.
Even in the PUG situation, if the raid doesn't have a lot of druids, and with the 30-minute cooldown we still have in live, a smart healer will probably save that Rebirth if its use doesn't look like it'll make or break the current fight. If you use it when it's not really needed, it'll probably be on cooldown for the next TWO boss encounters.
What the per-combat limit means is that you'll probably cause problems battle-rezzing whomever you want when it's not called for, unlike now, where your enthusiastic rezzing can only be a help.