This is not supported by numbers of people actually doing this. Resto shaman can't AOE DPS efficiently. Have some AOE in fight and it'll be behind a real DPS substantially.
Here are statistics on DPS of all classes at the 75% percentile, on all bosses:
https://www.warcraftlogs.com/zone/st...gregate=amount
A 75% percentile Resto shaman did an average of 1.4K dps.
A 75% percentile Disc Priest did an average of 8.3K dps.
A 75% percentile fire mage did an average of 23.6K dps.
So the healer class that is doing most DPS is a disc priest, not a resto shaman. And a 75% percentile mage does on average 16.8 times the DPS of a 75% percentile resto shaman.