So...in other words, you are saying that people should play a tank just to fill that void in the party requirements, they are otherwise unnecessary, and you could just run with 3 dps and 1 healer instead. Did I get that right?
It's no wonder nobody wants to play tank. DPS gets satisfaction from seeing big numbers. Healers get satisfaction from making clutch saves. What sort of satisfaction do you think a tank would get? If there's no satisfaction, why bother playing that class at all?
Saying a tank's ability has little to do with the success of a dungeon run is a very dumb statement. Everyone needs to pull their weight regardless of their role.
For your WP boss problem, telling him to gather the adds, instead of having the healer run in to aggro and Holy them, was a much better option. At least you are teaching the guy how to tank that boss for his subsequent dungeon runs. Having the healer do his job for him teaches him nothing.
Continue making statements like these, and people will be even less inclined to play a tank.