The question is...
Do they really want to go back or do they go back because they need to?
Having been laid off from my last three positions, I'm not 100% sure that I would want to go back to those companies. Of course, two of them are out of business at this point, so there's no chance there and the one that's still in business I think that I'd be looking over my shoulder the entire time waiting for the ax to fall again.
I doubt that they'd start me back at my old salary, give me my seniority, benefits, etc. back either, so really there wouldn't be a lot of incentive to go back for me. Sure there may be a huge benefit and cost savings for them as they get someone who's essentially been with the company for 6 1/2 years already, but I'm at the point in my career where honestly I want to know what's in it for me too because if I don't see a clear path up the ladder then it really isn't worth it to me to spend my time there.