I think you are overlooking this issue and you break the functionality of a site by adding various rewrite rules that all they do is hide a problem (if one exists)
First, any request may give a 200 header by adding various parameters to it. For example:
where values of i,j,k are say the spam keywords. Do you care if the server responds with 200 OK? No.
Second does the link http://www.example.com/index.php?i=1&j=2&k=3 propagates inside your pages? In other words if you view the HTML source after you access the site with that kind of link, does that link appears somewhere? Does it propagate? If it does then yes, you have a problem with the application and you need to find it and fix it.
Lots of these attempts are probing for this type of weakness. If one exists it can be exploited and not just to propagate spam.