There are two issues really. If people are able to vote and then instantly power down, it will cause fluctuations in vote values and rewards, much more so than now. The SP allocating rewards could change in large ways, instantly. That doesn't currently happen with the slower power down features.

Secondly.. someone could vote and then power down everything... and move that chunk of steem to another account and use all their votes and then power down instantly again. Then move that chunk to another account use their 10 votes etc etc...

The penalty for instant power downs would have to be large enough to prevent that move from being profitable.