Eduard - I agree with you to an extent. Yes there are going to be people who are going to try and cheat. Unfortunately that's the way of life. The only defense to cheating is to make it so difficult to cheat that it's not worth while. BTW, I think that a "Critique My Shot" is a forum we should have regardless of whether or not we have photo contests.
A bulletin board like this one should have a voting program built in, or obtainable, that reads the incoming votes and keeps the tallies. These voting programs have what is called an IP reader (or something like that) that reads the address of the computer from which the vote originates. All computers set up to operate on the internet have this address. They are unique to that computer. The voting program can be set up to read the incoming IP address and only accept one vote from that computer for that contest, so that someone can't create multiple accounts and vote for himself that many times. Not on the same computer at the same location anyway. If he was still intent on cheating, and say cast 12 votes for himself, he would have to find 12 different computers at 12 different locations in order to get 12 different IP addresses. On top of that he would still have to create 12 Nikonites memberships with all the fake information plus 12 different email addresses from each location. And this is for one contest. If he wants to do this for another contest he will have to go back and change all the fake membership info, email addresses and create new Nikonites memberships just to avoid suspicion. There are other things that can be tossed into the mix that will make it even more difficult to cheat.
While cheating is possible, because of the complexities and work involved, it is most likely very rare and not significant. What I suspect you have seen on other sites as alleged cheating, and ill will, is allegations from people who think they have submitted a much better picture than the one that did win and theirs didn't even place, therefore the winner must have cheated. The most common cheating that goes on in contests where voting is involved is calling up all their friends and relatives and get them to create a membership and vote for their photo. There is nothing anyone can do about campaigning for votes.
I don't think the possibility of cheating is a valid reason not to have photo contests. I do think there is a lot of benefit from creating the spirit of competitiveness amongst the members to improve their shooting skills.