But even looking at talents specs isn't always enough -- although it's a great first guess. Just like looking at aura/stances/form/etc. isn't always accurate. I know some fights we have prot pallies or druid tanks heal. As a holy paladin, I've even been asked to heal a fight with righteous fury up to try and draw mobs to me away from squishes.
Unfortunately, I think this problem is always going to involve some manual sorting.
GridDynamicLayout already does some of that, I don't rely on talent analysis because as you say, sometimes people are expected to perform a role that is not related to their spec...
That said statistical analysis feature can help the "people jumping around" impression by providing a more stable raid environment.