To start with if a programmer is creative then he cannot control himself but suggest UI changes, functionality changes and if too intelligent then expect nothing less than a brand new business model based on your old one.

Another thing that can happened to creative programmers is that, because they are creative they will come up with a product or an application and start working on (coding) it but in the process of development they might come up with another brilliant new idea and the old one is shelved and the new one starts to see some light on the other end of the tunnel but never comes out of it because now another idea is better.

Usually people who are creative also have a huge EGO because of which they do not like people finding loopholes or improvements in their ideas. This is another reason programmers should not be creative.

Programmers should not fall in love with their own ideas. If this happens then the programmer will do everything from marketing, planning, promotions to partnering and etc… But code.

This is the only reason why companies maintain separate UX design teams. This is why separate marketing and development teams exist.