As said elsewhere in this thread: Many witnesses power down constantly, including myself, to cover costs for running witness servers. I'm at a monthly bill of almost 1000 USD nowadays for steem-related server costs, for example. But, at the moment I actually power-up my powerdowns as soon as they are paid out, for the case.

Note that I don't power down my full SP, but just a small portion, and Im sure more people are like that.