Changing classes / specs between encounters
The best guilds in the world already send people to respec between fights or swap out classes between fights. We don't want this to be the standard way everyone raids, but we don't think it's worth implementing a lot of punitive measure to prevent either. So dual-spec won't affect those guilds much.

So then it comes down to what is most likely *your* guild (unless you are in one of those cutting-edge guilds, in which case I salute you). In your guild are all the players such amazing players that they can always hit the theortetical max dps predicted by spreadsheets and combat dummy tests? If so, then the pure classes should still be on top. If not, then you still don't have much to worry about. If you are an amazing mage and can out dps the shamans and druids, then you'll probably get a spot. If you aren't an amazing mage, but you have good friends in the guild and they like having you along then you'll probably get a spot.

The groups that might get caught are those that struggle with content and feel like stacking classes is the solution to get them through. But in nearly every case I see this, the group would do far better to just get better gear, talent builds, or learn how to handle the encounters or which buttons to push. The exception are the cutting-edge guilds that ARE struggling with cutting-edge content, but they are going to stack classes per encounter anyway (and 99% of us aren't in those guilds). (Source)