It is a good idea when you think of it that way. But then there are quite a few movies out there that are not good enough to warrant a 5 or bad enough for a 0. Yes, it is true most people vote 5 or 0, but thats usually for getting the blam/protect points and sometimes in the top50. It happens everytime an awesome submission comes into the portal, itll get lets say a 4.40 and then it makes it to the top50 of all time and its score slowly and eventually plummets to something like a 4.25 or 4.24. This is evidence of people voting 0 because they want their favourite submission at the top. But that wont change with the new rating system, these amazing submissions will still lose their superior score and become one with the others. Once a submission actually makes it to the portal, from what i have heard and seen in some reviews, people vote on in for how good it is because it doesnt matter to them whether its score drops or raises 0.01. And the last thing I have to say about the possible new rating system is that it wont be as easy to distinguish the amazing submissions from some of the worse ones. With only 2 options, and people see an ok movie/game what do they do? They may end up voting Thumbs Down or thumbs up and hence receiving the same vote as something horrible or something awesome when it really doesnt deserve it.
Basically I think this idea would be good for some things, but for the most part itll just make things harder to distinguish from each other. And there are still many people who use this current system the way it was meant to be used, by voting 0,1,2,3,4, or 5 accordingly.