My Dad has PC and his oncologist told us the same thing. He explained to us that the side affects of Tarceva are much harsher than Gemzar, and the benefit would not be that great in my Dad's case. He said that the goal with chemo is to give Dad a better quality of life - not quantity, and giving him Tarceva would defeat that purpose. (My Dad's PC recurred after Whipple May 31, 2007.)
Hope this helps. My heart goes out to you and your family!