This is one of the many reasons why I despise instanced raiding as an end game.
But your choices are really to play their specs or not raid. You've got 19 other people spending their time and consumables.
In other MMOs you can do a raid with 3 people or 100. It really doesn't matter. So there isn't the same level on min/maxing. And there's also PvP so only a small part of the raid would want to be glass canons any way.