Comment 3 for bug 1255075

You didn't. It is just that you either install a plugin or you don't. If you want a plugin sometimes and sometimes not you should use a plugin manager. Setting this one variable will only disable parts of UltiSnips, i.e. the ftdetect hook is still installed. Vundle will disable it completely.

Vim's documentation is very old - a lot of the best practices in there are no longer true.