I think the fake UDIDs were introduced to facilitate privacy. It’d be kind of annoying if you uninstalled Facebook, then reinstalled it later and it automatically logged you in. A fake UDID is still useful for tracking the lifetime of an app install, though.
If you have testers who are willing to install Xcode (7+), Apple has now lifted the restriction where you had to have a paid developer account to install apps on your own phone. So you shouldn’t necessarily have to have your testers use any of the slots on your paid dev account.