I think we need to make a clear distinction here:
Developers should NOT be writing PHP that will require register_globals to be turned on, and that will be safe if it is.

Sysadmins should try and run all of their PHP apps with register_globals off in case there are PHP apps in use that are vulnerable with it on. If necessary, it should be restricted to specific directories via .htaccess.

It's really too bad register_globals couldn't be set at runtime via ini_set() as that would mean instead of erroring out, apps like Slarty has written could disable it for the duration of the script. I understand why this is likely impractical, but it still doesn't make it less useful.