Oh, that's definitely true. The Engineer can be very powerful, but in the hands of someone who knows how to play it, and how to use the kit switching and all.
It's a bit like the Ritualist in GW1. The Ritualist was a true jack-of-all-trades. He was able to heal, protect, do armor-ignoring spikes, pressure, AoE, and in the right circumstances, even tanking. However, you had to know his limitations and the "flow" of his abilities to truly make it shine. The Engineer in GW2 is pretty much like that, in many senses, which is partly why I'm so excited about the possibilities of playing an Engineer.
So yeah, for healing support, the "specialist" class would be the Guardian. It's a class mostly built around support, so it's definitely easier to play in that role than any other classes, water-attuned Elementalist included. But I can see Elementalists and Engineers taking that role on occasion, when it's needed. And don't forget the element of surprise - when someone sees a Guardian, they know what to expect. When they see an Engineer, they'll lose some health/time finding out what he's up to. ;)