If you're healing as Summoner to the point your dps is suffering, something is very wrong! Either the run is a total disaster, or you're not trusting the healer to do their job. It is normal and perfectly fine for a tank's health to go to 50% before a healer starts healing. If you're throwing your physicks on there too, you're backseat healing the actual healing role and wasting mp you could be using for dps.

If you want to be a pure healer I really recommend switching to WHM until you unlock Astrologian in Heavensward. AST is the job least expected to dps, imo.