I'm bound to say that of course I unvote witnesses from time to time, and I think witnesses actually try harder to do their very best if they can't take votes for granted. :)
Like I prefer manual curating over auto upvotes I also prefer 'manual' witness voting instead of choosing my witnesses once and then just stick with them forever.

However, I agree that every Steemian should make use of his right (and 'duty') to vote for 30 witnesses.

That said (and that's why I comment here at all), I think one single account should only be able to vote for five to (at maximum) ten witnesses!
Otherwise a big stake holder like Justin Sun could be able to control the whole blockchain. And it doesn't really matter which percentage of STEEM power he owns now. A person who spends more than four million dollar for a lunch with Warren Buffett would be always able to buy as many STEEM as necessary ... :)