There is a drawback to this, obviously. If your codebase is some scattered mish mash of Python, PHP, Actionscript, Javascript, Java, etc, your ability to reuse code diminishes at the language boundaries. You end up having to write very similar code to deal with the same objects on the client side and the server side in multiple languages, producing more code to maintain overall.
Unfortunately because of the nature of the web I haven't seen any real good solution to this yet. At minimum, you're usually dealing with some combination of Python/Ruby/PHP backend and some amount of Javascript/Actionscript on the frontend.
True, though I don't want to leave you with the impression that it's a mish mash implementation. It's more of a silo'd approach. One section of the website might be a wiki, so we use a particular tool there. It's a self-contained unit but perhaps pulling in some common stylesheets to retain a consistent look and feel. Another section might be a blog part of the site. Again, it might pull in some common assets for the consistent look and feel.
Instead of thinking of code re-use, we're left with a UNIX style resource re-use. Every component can stand alone, but they all work in conjunction with each other and can be re-arranged to work in different ways.
Unfortunately because of the nature of the web I haven't seen any real good solution to this yet. At minimum, you're usually dealing with some combination of Python/Ruby/PHP backend and some amount of Javascript/Actionscript on the frontend.