When levelling my SMN sub, I was main healer about 70% of the time. The rest of the time I shared healing duties with another mage (RDM + SMN was my favorite combo: both can do 50% healing responsibility without breaking a sweat and still have time for all the other stuff they want to do).
Never did I have a party where all there was for me to do, was use my avatars. Nor should I have: such a party would have, frankly, been better off without me, since it would've already been fine on healing. If a SMN's *only* role in a party is to DD (well, pre 65/70 anyway), other jobs can do that much more effectively.
Note, for what it's worth, that this was before BPs were split into Rage and Ward. I haven't played SMN again since that change.