Ultimately the delta shield was adopted for the entire starfleet after the Enterprise was the only starship to survive the 5 year mission intact. Don't know how accepted that is but it is my understanding.
It's a fanwank. Make of it what you will.
The return of Kirk from his 5YM, with his ship and crew "relatively intact" is from Roddenberry's own TMP novelization.
The ultimate in fanwanks, perhaps?