Also, it may be to prevent people from voting multiple times with transferred stake. If with a 30 day payout period 3 months was necessary, surely with HF17 and 7 day payout the power down time should come down to 3 weeks. I agree with @snowflake, a lock-in period of any kind is bad PR.

None of the top cryptocurrencies have any lock-in, yet they are successful and relatively a lot more stable than Steem.