As JC pointed out in his first post. Its depends on the mob. Also take into consideration the number of players in classes. If there happens to be a lot of shammys and few druids, a Shaman can be in a heal chain. There is no set-in-stone rule.
don't have numbers offhand, but quies is certainly more efficient than a ch, and depending on your focii/aa, blast shammy heal > shammy ch.
but efficiency isn't the largest problem, shaman ches just don't heal enough. 2k just isn't enough to take a spot in the heal chain on any
encounter where you actually need a heal chain.