Usability and Design are two sides of the same coin. A designer who is able to look at design from both the usability and design angle tries on the one hand to prevent a faceless and aenemic outcome that might be the result of very strict recommendations and other constraints. It is his duty to bend the rules a bit. On the other hand he must still respect the "laws of usability" to the spirit. And in order to be understood by the Usability Expert he subsequently needs to motivate his choices also on the basis of the principles of usability. A defence based solely on aesthetic arguments is not valid.
Through regular co-operation with different usability experts and through experience I have developed an almost intuitive grasp of the principles of usability. On projects that did not have a usability expert I have often de facto acted as ergodesigner, first creating the wireframes if required. The advantage is that I can easily communicate with Usability Experts and that I can interpret their input in a correct way and take things to the next level. Another advantage is that I can propose valid alternatives if I feel this is necessary.
In short I can act as a hybrid usability/design expert if the project's budget doesn't allow for the presence of two separate experts.