If you're looking at it as an investment, then yes, you should be powering up at every given chance. The idea is to grow your influence.. and eventually also to benefit if the price of Steem shoots to crazy highs like with Bitcoin.

It takes 13 payment instalments as a safety measure and also to prevent a suddenly exodus of steem - imagine if a couple of Whales decided to power down at the same time. That would negatively affect they platform if they all the money was withdrawn immediately