Oh I was talking about casual content in general, regardless raid or dungeon. Well I wanted to stick with the topic of raid, but I never really have a firsthand experience where the tank is missing the whole fight, and I only have the experience in boss dungeon thus I mentioned that one. Honestly I really would like to see how a party of 6 without tanks clear a casual raid. If the 6 ppl are competent enough to carry the missing tank weights, I think it should be doable. might make the healers raise a lot & throw GCD heals a lot though, but should still be doable.
Regarding the holy trinity, I personally think that roles are not very important. The casual content would do just fine without having the holy trinity of tank-dps-healer. but it is helpful for the very casual player who don't really know how to play the game well, since having the normal party composition does make the content easier. For most hardcore contents, the roles matters a lot more though.
Talking about some of the hardcore contents that can be cleared without healers, well maybe those ones need some balancing fix. the game is not perfect, but I think the way 2 tanks & 4 DPS manage to clear a casual raid provided those 6 are skilled enough, is nothing to be concerned about.