Yes, and dots tick every 3 seconds. 500 damage every 3 seconds is 166 DPS [Damage Per Second] When you think about it that way, 500 dot dmg really isn't a lot.
BUT I do agree that SMN dps is not as terrible as it seems, just there sustain. Case in point, my Coil static SMN was pulling about 520 on Cerberus until he got killed xP
Its resource management, as has been stated many times before. TP-based DPS get Goad or Paeon. Very rarely will Ballad be played in Coil situations, but one of those 2 TP abilities will always be given.