In the period between receiving your liquid steem payout and choosing to power up again, there will be new blocks and new steem allocated to the vesting pie. Thus when you choose to buy back in it will be slightly more expensive, you are paying an opportunity cost by always powering down, but you are paying another opportunity cost by not doing so.

This generally isn't likely to be a significant amount, but if everyone was doing your strategy, the few people who decided not to power down, would receive a significant share of the newly created Steem in the Vesting pie at those times.