Just came across a great quote in Alan Cooper’s book About Face 3: The Essentials of Interaction Design. The quote perfectly sums up why programmers cannot do design when they are programming:
[...]building an animated status display into the face of a program might require a thousand or more lines of code. Programmers cannot be expected to make the right choice in this situation. They have a conflict of interest, so designers must be sure to specify precisely where information is reported on the surface of an application. The designers must then follow up to be sure that the design wasn’t compromised for the sake of rapid coding. Imagine if the contractor on a building site decided unilaterally not to add a bathroom because it was just too much trouble to deal with the plumbing. There would be consequences.