Remember, with Gift of the Earthmother, you drop down to a 1 second GCD on Lifebloom, meaning you will have a lot more free time to cast other spells while topping off tanks. Naturally, if you're healing 4 tanks at once, you need all your concentration for LB/RG, but if you're healing a single tank, you have tons of GCDs to spare.
I rarely get a raid healing assignment, but if I do, I tend to prefer Regrowth or RJ/SM for dealing with big spikes, and only go with Lifebloom if I know the target won't die before the bloom goes off. Rolling LBs on raid healing targets is rarely an effective use of cooldowns, except in special situations like Bloodboil (and even there it's impossible in current mechanics to keep all 5 up with LB unless you have a lot of haste). In point of fact, in 3.x, I'd drop Flourish on the Bloodboil group and call it a day - a single Druid could keep up all 3 groups using this ability, mana issues notwithstanding.