A click castsequence really helps, when you're running multiple toons of different classes.

While it might not be optimal, it is a lot closer to optimal than manually managing (impossible really) several casters of different classes at once.

If your team is 5x Boomkin or 5x Shaman, then you can have them all Flame Shock or Lava Burst together. If the team is Boomkin (who wants Insect Swarm/Moonfire, when not up, and Starfire/Wrath when they are up) and a Shammy (who wants Flame Shock, then Lava Burst on each cooldown, then Chain Lightning on each cooldown, and Lightning Bolt fall-throughs)... it is much easier to spam a single button.

Whether the sky is falling or not... We cannot be sure if a change will be permanent in Cataclysm or not. Whatever they do, we'll adapt as single boxers will as well. Our advantage will still be coordination and our disadvantage will be a lack of simultaneous different actions, compared to one-boxers or five single players.

It is too early to say whether this is a change for Cataclysm or not.