I must say I can't understand anyone going all the way to the polling station and then wasting a vote.
That's the only way to make a valid protest vote. The other alternatives are "couldn't be bothered" or "what day is it". As seen on numerous street interviews on the news. No one said "not bothering was my way of protesting".
On the face of it.... to me, it seems cameron thought it might be a good idea, if he got a good number of conservatives in the police commissioners seat, to boost his ego, when the local council was returned to office under a Labour or Lib dem, vote.... and , of course, if the police commissioner is conservative, then they will follow party line.... and not necessarily the line of the local council, or the local people.... just another way of getting inroads to local politics and being able to control them from conservative party offices.