I understand the reasoning behind this, but this won't entirely solve the problem.
* Developers of free apps will be a lot more likely to have unwarranted negative reviews. All I have to do is download a free app, and then I can say anything I want about it, whether or not I use the app or not. It's a lot less likely someone is going to download a $100 app just to trash talk it as compared to a free or $0.99 app.
* As has been pointed out, version numbers and dates of posting aren't indicated in reviews. So as a potential buyer, unless I have been closely watching a particular app, I have no idea whether or not a crashing issue was a iPhone 2.0 system issue or an app-specific issue.
That said, I have noticed lots of negative or nonsensical reviews. I'm assuming Apple doesn't have the manpower or desire to just delete all the spam reviews, so I guess the purchase/download to review requirement might be the best compromise.