The main reason is probably to incentivize locking up coins in SP, which is is needed for a variety of security reasons including reputation but even double voting (if you can instantly move your coins from one account to another you can vote again with the second account).
If you do lock up your coins you don't suffer from the crazy inflation, so as I commented to iamnotback above, you can't really count both as disadvantages. Pick one or the other.
A much shorter luckup period would probably still solve most of the problems. I didn't design the thing
