As a raiding mystic I have them up all the time.
I usally ward, then debuff, then heal ect... but i always debuff and if there are multiple epics in the encouter then I will maintain single target debuffs on each epic. Heroic mobs i tend to just keep my group slow and group sta and mit debuff on them.
Honestly If you are playing a mystic and you are not debuffing then you are only playing half your class.