The engineers were even considering using it for a high altitude satalite launching plateform. It really is sad it was not developed. But we do know the same engineers took their idea's with them and went to work for NASA and such, so who knows just how much of their brainwork is in actual use today.
I'ver seen the specs for the Mark III Arrow and it would have been a heck of an aircraft. The US produced practically nothing to compare until the F14/Phoenix systems.