Given the cost of these films, I can't understand why more companies don't use historically accurate props.
Things are getting better in that regard, though, and the Patriot is a good example.
Consider "Patton" where the German tanks were obsolete American tanks with crosses painted on them and the Americasn tanks were'nt even Shermans but Chaffe light tanks not even in service at the time, and "Saving Private Ryan" and "Band of Brothers" where the German tanks were completely convincing even to a trained eye and the Shermans were real.
The average person doesn't know the difference, but these thing make the movie for me. Or a least make a good movie even better.