Not very effective, and not very "supporting" as their role is supposed to be.
You don't see them because other classes do a better job at "supporting" such as psi for debuf support & bard/engineer for buffing support.
Since we're limited to 2 builds only, most - if not all - players run a dps build for pve & dailies, and run a heal build for pvp/pve healing, since it is currently the best class for healing.
Its because most ppl dont know how to Play it, its one of the strongest atm if u were to 1v1 or bg´s, its dps is mediocre + debuffs´but it has one of the best survivabilty of them all.
Thats only if u are at a competetive lvl with a Team, generaly speaking when ur alone and queue up for stuff, support warden is one of the best, its only when u have a team and try to be competetive it is good as FH cuz other classes brings better stuff than a support warden