It depends on the application that you are working on. The more you rely on geographic information the more strict you must be. So let's review why you need all that info. If you don't include the zipcode with all the rest of your information, you run into the Washington Township problem. http://en.wikipedia.org/wiki/Washington_Township

You can't infer (reliably) your exact location from a zipcode because of the reasons listed in the other comments linked to your parent comment.

Because zip codes do not equate to cities 1 to 1. Many cities have multiple zip codes, and some zip codes span multiple cities.

Zip codes are based on postal routes, cities are not.

ZIP codes can be shared among multiple towns. Quite possibly multiple states, though I can't think of an example offhand.

It's less work for the developers, who might have more important things to do?