Let's face it: american companies are not productive over thanksgiving and christmas. They just pretend to be. By making these 2 weeks vacation, we simply embrace reality.
Or maybe the 2+1 was for the first year you were a company, as opposed to individual's tenure?
1) The 1st year at dotCloud is a big step for us, and a lot of people didn't make it in the past. Making it means that somebody has truly proven his worth (since he or she wasn't fired) and has adapted to our unusual culture (since he or she didn't resign). By attaching perks like this to the anniversary, we emphasize its importance.
2) We don't want people to join for the vacation. We take a lot of breaks because that's what intense, hard-working people need to avoid burnout. But the majority of people on the job market are not hard-working and intense enough for our standards - we're hoping this will help us filter for them.
I'm not 100% certain this would work in a bigger company where people think of it as "just a job". But in a small team where everybody is passionate and eager to make it successful, it works great.