It's not just that some people want to vote for the winner, but if a certain option is already guaranteed a win, I've voted for other options that I think need more votes (another reason why is illustrated in the last part of this thread). Hiding the running tally also means there's no advantage to waiting to see what the voting trends are, and it will give a better indication of the actual sentiments of the voting majority.
Of course, this could put a couple of pictures at a disadvantage, if there are some similar images (such as the 2 submitted by MikeR C), as some people may want to only vote for one, and may have chosen one over the other based on voting trends, but instead they may end up scavinging each other's votes. That's the main reason behind having only 1 of any given cannon submitted. Mike's submissions don't appear to violate this requirement, but again, are too similar to each other IMO to garner a vote for each, given the rest of the options.