Can we just clear one thing up. Are you talking about parsing .html documents to run php in them? Or are you taking about using mod_rewrite to rewrite page parameters into a static url?
In the case of the former still cannot see any benefit from doing it, just a false sense of security imho. As for the mod_rewrite solution this can be effective. As you probably know when rewiting the urls you have to write a regular expression to match the incoming data. As long as you write these rules sensibly these can block incoming attacks, as the invalid data will not be matched and thus ignored.
I still maintain my belief that just changing the page extension offers no real protection.
As for the advantages of using the .php extension over .html - there aren't any it's just the norm.