I think it's always up to the guild to decide how they are going to handle raids. If guilds allow full solo, then g12 doesn't matter. If they have other restrictions, I don't see why they either can't restrict g12 toons or the person can just pull out after a certain amount of damage.
The raids start off difficult and get easier once you get stronger. My guess is that guilds that find Heroic tank easy will be occupied with TB for a while, until the next thing comes along.