@ned Powering down at half the rate and losing half voting power means doing so for twice as long to power down the same number of coins. There is effectively no difference.
Mathematically it works out to be an exit fee equal to 1 year of voting power (and associated rewards) for each coin powered down.
Do not think for a minute that the presence of such an exit fee won't deter and discourage entry, because it will, in addition to having extremely negative PR consequences. This is a terrible idea.