What if they'd just release their rendering engine, with a very simple UI which only lets testers enter a URL? After all, most of the problems are in IE's rendering engine, not in its UI. That would solve the problem of journalists etc. looking at it as a real product.
Now, I do doubt the usefulness. We can't improve the code like we can with open source projects. Giving feedback about the rendering engine isn't all too useful either, because the IE team cares about standards nowadays and uses many tests themselves (W3C testsets, Acid3, CSS3.info). They already know the bugs, so the only thing we could conclude with a nightly is how far along they are.