The inflation of Steem is the real reason why keeping tokens as Steem Power is useful. That said, Steem Power takes 2 years to power down, so it's not nearly as liquid. @dantheman has definitely set up the correct incentives since Steem Power helps you upvote stuff.

Also, I'd be lying if I sad that I understood how things Power Down. There's some Steem Power to Steem conversion rate which I still don't get, which has incentives as far as how much people keep in Steem Power vs Steem. Anyone know?