That would only make it longer to find a group.
Ok, it is annoying, to have a second drg, nin or bard in the group (dont remember, if there are other classes, who overwrite the own buffs). But in overall would this wish only create more problems.
Its esspecialy bad in times, where one class is overused. Like, new DD classes (at the start of each expansion, where nearlyneveryone are using the new stuff).