My mistake, I thought they were 3+/4+. The average in the same. Tree-revenants can pile in 6, instead of 3, as long as their banner is present. A 20 model unit of dryads averages 6-7 wounds against AS4+ on our turn, and 5 wounds on the opponents.
My point is that they offer rend, which is usually at a premium (treelords, hunters, alarielle, drycha, all at or above 200 points) for us, and they tie up opposing units even if they're not engaged with them. Shooting units suddenly need different screening. If they use a unit to screen the front and one side of their shooting unit to prevent us from trying to push our revenants into that unit, that gives us the change to send our dryads, hunters, Durthu, etc. into that screen and take less damage back. If they use two units to screen different sides, then we got our opponent to effectively remove one or more of their units from the game without fighting it. By having a unit with a huge threat radius sit in a corner or on a safe backline objective.
If they don't, we can try for the charge, and even if we don't kill all of that unit, many shooters have 5+/5+ melee profiles, and we limit their shooting to just the revenants for a turn while our more important targets don't get shot to bits and get healed.
I'm not arguing that dryads are worse, just that revenants open up options and lines of play that our other units don't.