Is there an advantage to dynamically loading/unloading javascript and css stylesheets?

Once your javascript or css file is downloaded to the machine, it is cached by their browser. So, you don't incur the additional cost of another HTTP request. Lazy loading the scripts and stylesheets could make sense, but there is no sense in unloading these web assets once they have already been sent to the client.

It is good to use some sort of mechanism to compile your scripts and stylesheets to minimize the initial http requests to one per asset type. Having only one stylesheet and one javascript file would be an architectural nightmare in many cases, but that doesn't mean that you can't still present it to the browser that way. I use .NET, so I'm not sure how you handle this in PHP, but I'm sure the functionality is out there.