I don't think this is the argument. I mean, yeah alright it won't result in a wipe in Void Ark if one of the OTs is in his tank stance the whole time... but that can also snowball into other things, like "1 DPS AFKing isn't that bad", "We can do this without 1 Alliance", "We can do this with just the 8 of us", etc. It can also result in aggro ping ponging all over the place, which could result in all of their deaths, which would inevitably lead to everyone else dying. As an example, back in WoD I recall tanks bouncing hate around and getting everyone shot with the laser eyebeam... multiple times... until we wiped.
Anyway, I'm fairly certain the argument is that one of the tanks should've attempted to get aggro back and once aggro was settled they could all go back to their business as usual. However, none of them apparently turned on tank stance and instead just let everyone burn.
I don't think this is a good mentality to have in a raid scenario. If you are in a situation wherein you can be in DPS stance and that results in a net group DPS gain while not compromising your survivability, then you should be in DPS stance. By not being in DPS stance there you are making it more difficult for your group to meet a DPS check or push a phase faster. As an example, if you aren't in DPS stance during the HoP check in A3S then you are putting massive amounts of weight on the DPS' shoulders when it's unnecessary. You are a part of a group, and to kill a boss you need to make it's life bar reach 0 - which means that if anyone at any time can push DPS harder in any way without compromising other things, then they should. This is not always the case, there are instances where tank stance is beneficial and in previous raid content (ie T12, T13, etc) you needed to be in tank stance to reasonably survive the tank busters, and in the case of T13 (in progression) you needed it all the time.
But yeah, basically, if you can push group DPS without getting yourself killed, you should and by choosing not to do it then you're only hurting your group. This is the case for every job, including healers. A lot of people don't like it, but a lot of people shouldn't be raiding.