If they were a legal requirement then why weren't all cars fitted with them? None of my previous 60's cars had overriders fitted apart from the Jag's & Daimlers.
I'm not sure it is illegal to completely remove bumpers, either. It would be illegal to remove them and leave the mounting hardware in place - you would get pulled for "dangerous parts" but not for the absence of the bumpers.
I would have thought that getting caught on the knee by an overrider would do more damage than a plain bumper.
All this said, I think I prefer the E-Types with overriders fitted.
Regards