Well, the main reasons for not converting .html to interpret as PHP are twofold. First, programmers don't expect .html files to be run through any sort of script processor. If someone else is ever working on your site, they might come across this behavior unexpectedly, with some potentially bad consequences. Second, since .html files are static, they can be served up very quickly and easily by Apache. It takes a lot more overhead to run the PHP interpreter on a file than it is to simply throw the file down the pipe.
These reasons are augmented by the fact that there are other ways to achieve the same results on the user side. Apache's mod_rewrite will let you map any .html file request to a .php script in a way that's completely transparent to the user.
Also, you mention Wordpress. Are these .html pages cached posts that Wordpress automatically generates? If so, I think you can tell Wordpress to regenerate these pages when you change your link structure, so you shouldn't be losing too much. But, I may be wrong on this one, since I've never had a blog busy enough to warrant using a static cache, and have therefore never tried it myself.