Am I the only one who thinks good application developers should also be decent designers? Why does everyone assuming that a person must fit into only one group?

Being a good developer is not about typing code, it's about creating a great application. That means using all of the tools at your disposal... code, visual design, workflow structure, analytics, etc.

In my experience, traditional designers often do very poor work when it comes to interactive applications. As a developer, it's my responsibility to tell them it's crap, why, and present a better solution.