It isn't a matter of it being an annoying habit, it's a matter of it being a different technology. PHP has a way to include pages, Frontpage has a way to include pages. If your client wants to use FP, then it's probably best to use the FP technologies otherwise the client won't be able to use other FP features.
Now, you can use PHP in FP, just configure your .htaccess to parse .htm or .html pages. Grab a copy of the PHP Rocket plug-in for FP and you can develop on your local machine. But then you'll wind up using FP as just a handy-dandy code editor, defeating the client's purpose.