Choosing "don't be evil" as a credo deliberately encourages others to view the company through a moral lens. It is an invitation to judge the company according to a higher standard than most businesses would hold themselves to. It makes explicit that the company takes responsibility for the moral implications of what they do instead of pretending to live in an amoral value-free universe like many other corporations do.

I think it was a courageous motto and I'm sad they dropped it.