90 days is about the minimum time period that prevents moving stake between accounts voting again on the same posts for significantly higher payouts. Other ways of preventing this carry a lot of overhead or other undesirable effects. This could be an issue again if the voting period for posts is ever extended beyond 30 days which many authors would like, but in that case other solutions will need to be found.
I agree about rewards in SP/STEEM. I don't think it is necessary with only a 3 month power down period. IMO that change could be dropped to make this simpler. Witnesses/miners would still have much faster access to pay to use for expenses.