You can't compare Healing Rain and Effloresence. For one, Healing Rain costs a shaman about 10k mana. Effloresence for us is free.
That is the basic reason i took Effloresence, it is free healing so why not? I dropped Nature's Bounty in order to take Effloresence (still have NS/Nature's Cure/BoTG too) As Regrowth is used sparingly (only really with CC procs i didn't see much benefit from the crit it would provide especially after the CC nerf.