Haha, it always cracks me up with the Healers do DPS thing. Honestly It goes like this in priority.
1. Keep Party Alive <> get rid of negative status'
2. DPS
Number 2 is highly dependent on a few factors.
1. Comfort- When you don't feel comfortable stance dancing etc... thinking you will miss a critical cure when it is needed, you are going to be less likely to want to gamble with the DPS side of a healer roll.
2. Knowledge of current Content - Knowing when and where it is appropriate to be able to dps takes time. Just because you have healed this dungeon 358729587 times doesn't mean the healer you have, has... this ties directly back to number one.
3. Experience with compositions - While having different compositions generally turns out the same, there are several nuances to learn, that significantly change the heal/dps "when, wheres and hows"
4. General reaction time of the player - I know it is hard to believe, but people are not all equal with the speed in which they are able to react, this is especially noticeable on healer. I don't think they should have to switch rolls, because they are playing what they like.
AND at the end of the day ... lets be honest. In general dungeon runs... a healer just healing... isn't going to wreck the party. A healer missing critical heals and timings because they are trying to juggle too much when they aren't comfortable yet... will. This isn't to say you shouldn't try to do this whenever you can. This wave of "healers that don't dps are crap" however; I don't think is called for.
In regard to to the original post, in my opinion, this is a crap idea. All it does is bling people that are already good at things and crap on people that may not be up to the same caliber. A rating system at the end of a run would be better, just to get an over all top down view of how people performed, but even that is "meh"
You always have the option to drop out of a group if it is annoying you that bad. I know there are some stubborn mules out there that really don't grasp the roll at all. In my experience however; this is far and in between, maybe I get lucky.