I think the Savings feature should definitely be improved where it's possible to choose how long a person can "Save" their funds.
AND
I think a person should be able to Power Up their funds as short or as long as they want but Voting for Witnesses & the #SteemDao should be reserved for people who Power Up for 13 weeks or more to prevent Exchanges from using their clients funds with no skin in the game to vote.
I agree the savings feature could be improved, but I disagree that extending the power down time would really affect exchanges that wanted to vote anyway.
Exchanges have to hold a lot of coins in cold wallet type setups while keeping some in the hot wallet for daily transactions. A large exchange could hold millions in SP while ensuring they have enough liquid on hand to handle the daily transactions. They could start a power down to increase their liquid, but if they have enough on hand, they can still power up, vote, etc while having enough to service their customers.
A simple way to analyze this is to look at extremes.
If there was a 1 second Power Down time exchanges would probably power everything up.
But if there was a 10 year Power Down time, they'd probably Power Up much less. So I don't think it is about the Power Down time not influencing them from Powering up but by how much.
P.S. I also didn't advocate for increasing the Power Down time but I want it to stay at 13 weeks.