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.