An enhance shaman can have it's benefits
they bring the raid a total of 475 extra Ap from windfury, 100 extra Ap from strength totem , 50 extra agility, the ability to twist between WF/GOA to keep both up with the 9 sec pulse, their own dps , Nightfall procs, (all of these are things Resto does not bring to the table)
They also toss off-heals, Tremor/grounding/FR totems ect., off heals , and wipe recovery.
Enhancement is often judged as solely a dps class, but it's a utility class and their own dps is just part of that utility along with greatly buffing the melee group and providing nightfall.
Should you run around and bring 5 enhance shamans to your raid? Hell no
Do they bring something to the table that makes one viable for the top melee group? Yes in my opinion