Healers are expected to dps now in WoW, it is wildly acknowledged in the community. In fact, they pretty much all have a fully fleshed out dps kit. Disc obviously, rdruid with dots, evoker with breaths and variations of flame (store heals which turns into a massive burst). It is the mark of a good healer to dish out dps while satisfying the much more stringent heal needs. Even the MDI (official pve competition) have healers do crazy dps, and sometimes be replaced. And that's much more challenging than any ultimate will ever be.
Besides, only the japanese seem to like this bs of a rigid trinity, and yet as we can see, other jobs are getting more and more support tools, while healers have 0 new dps tools.