I really don't know enough about the spells cast by shaman or especially cleric (still don't really understand how vitae works) but as a Warden I'm fine as the main healer. But what I will say is that as a Warden in a group with another class healer I prefer the secondary role since my experience is that our main line of spells (regens) work better when cast after a ward or reactive. The shaman and cleric essentially stop the tank from taking damage and then our more efficient regen spells (opinion) will bring the tank to full. Rinse and repeat.
At 101/tick (maxed regrowth at adept 1 with higher lvl HoTs broken), our regens are not sufficient to keep the tank alive so without a shaman or cleric, Nature's Caress, Effloresence, etc., need to be used more heavily but they don't seem as efficient.