There are badges available specifically for people who get a lot of their answers accepted with zero up-votes (Tenacious and Unsung Hero).

If the system were to change as suggested, to automatically give an up-vote to an answer you accept then those badges would have to change as well.

(this might not be a bad thing, as the current position does make winning those badges somewhat hit-and-miss, depending on the intelligence of the people you've tried to help)