Without the slash, an empty variable would result in a command line of "rf -rf" which would simply fail due to the missing argument.
There is absolutely no need for having a trailing slash, it's not as if "rf -rf foo" and "rm -rf foo/" can ever mean two different things, there can be only one "foo" in the file system after all.
Very interesting way of introducing an epic fail with a single character, that really looks harmless.