Using overdrive in the City?

Being that my transmission just went out at only 83k miles... i did a little research and asked around on when it is actually a good time to use overdrive... some people say its fine to use in the city, others say its bad for it... everybody seems to be 50/50 on it. my question is... if overdrive is so bad to use in the city, why would (insert your car company here) have overdrive be on every time you turn on the car?