- More humans available to identity bugs and undefined behavior

- Innovation in the form of programmers improving software on their automobiles and sharing their modifications

- Regulators are no longer the only group (other than the maker) with access to source code. This allows new entries into the automotive space without (badly) re-inventing the wheel. This breeds competition, and is a win for consumers.

> There should be no room for secret functionality

?

Unless, you could somehow prevent competitors from stealing I+D. Which you can't. So closed source it is (that makes things harder for competitors).