In more closed source languages like .NET you won't have the copy-paste problem nearly as much. In my time learning C# I found a great lack of freely available online resources with code snippets that worked in all situations as with PHP. Picking up PHP and making a site work was much much easier with the wealth of reference material that was around without costs associated with them. I would suggest that the copy-paste pitfall you mentioned depends largely on the language or platform that is being interviewed for.

Writing code without being in front of a computer is different in that you don't expect your napkin application to run and you won't list include files or even real functions in outlining a potential plan, does this detract from the usefulness of the napkin? No I wouldn't think so as it gives your brain a predetermined map to try and follow when finding the real functions to use or loops to create when solving an outlined problem.