Wouldn't this mean it's better to just give people one dose?...
I don’t know, but perhaps the immunity from a single dose isn’t a long lived?
It’s also possible a single dose is only 70% effective vs 90% effective; however if you don’t give people the second dose in a timely matter, you lose the possibility of giving them two doses later because of vector immunity, although I don’t know if that’s a thing with mRNA.
My biggest concern with getting a vaccine now is that it may preclude from future improvements.