Usability testing

Nowadays we heard more about Usability Testing. It is getting popular for 3-4 years. It is so much important due to evolving new web technologies. We also planned to do Usability testing in our web based application. Mostly we targeted the UI consistency, navigations, objects alignment and colors.

Definition
Learning
Samples

I read few articles about it and did testing. Another worst part is, your development and product engineering team should have more interest on that. Most of the usability bugs are considered as low priority if functional testers raised. Few of them marked as high priority after clients reported. Here tester's pride is getting loss.

I have a question for you. Try to answer for following question.

Application that has GUI must be tested for usability - Choose the right answer.
a) For any kind of UI applications.
b) More necessary for web-based applications
c) Only if the user is going to use the UI.
d) Only if the user pays for it.