"Many of the Ruby version switchers work with '#!/usr/bin/env ruby', but rbenv won't in certain circumstances, so an optimization for it has been built into the framework now."

Incorrect. It is support for an enhancement that rbenv provides.

You often want to invoke an application binstub from another program such as cron, and you want to use the per-project Ruby version specified in the application's root directory. Other version managers require you to spawn a subshell and load the version manager into the shell or otherwise set up the per-project Ruby version environment, then `cd` into the application directory, and finally run the binstub.

rbenv requires an environment set-up step too—set `RBENV_DIR` to the application root—but provides a wrapper that eliminates the need for such gymnastics. When you change a binstub's shebang to `#!/usr/bin/env ruby-local-exec` rbenv automatically searches up the binstub's directory tree to find the right Ruby version.