It works as a spam button. If I see stuff pop up on my guys I just spam F12 and if I look at my priest I will see his target cycle through all the party members. The target does change during GCD but if you mash this 10 times in a few seconds it wil target and dispel all your guys at some point. Then you can get back to nuking/healing.

Same thing with my shamans if I see a buff pop up on the boss then I spam F12 and they /assist /cast purge