Remember that all airline loyalty systems are designed for the "constant" frequent flyer, not the "newly" frequent flyer. They're meant to give you a better experience overall, not on any particular flight or set of flights (including the relatively small handful early in a calendar year before you have acquired status). So if you are going to be flying a lot in 2010, you're only missing out on a couple of months worth of status.

If you were flying a lot already but just not on Alaska, it's common to request a status match on the new airline, which avoids this issue.

The accounting to deal with a rolling 12 month year has got to be enormous (you would then have to account for rolling expiration dates as well) and might turn out to be unfair (or a lot more complicated) when any promotions like DEQM come out. Sticking to a calendar year makes it simpler for everyone, even if that means it's not exactly optimal for your particular situation.