Except when you get back to that "same old" and now have to make up for the month that was lost!

It's not interesting from my perspective. It's a good idea. Many companies could benefit from it. Not Valve, but many companies.

From what I've read of the origins of Portal 2 (and I can't provide a link citation, it was in The Final Hours of Portal 2, a long form piece released as an ipad app), Valve did something very similar to this, all the employees went away to work on whatever ideas, came back and presented them. The coop model with two robots for Portal 2 came from that, as did a non-portals gameplay device that evidently got them excited enough to start building Portal 2 around, until they realised they couldn't do a Portal sequel that didn't have portals.

What do you mean not Valve? Not Valve because they're already doing it every month of the year?

Yes.