You have to understand that I'm coming from the point of view of a cynical marketing guy, which is my wonderful day job. ;-)
But I will concede that marketing trends for conventions and such are skewed a little differently than those in the "normal world". I guess the situation would depend on the prices of your books and the quality/size of the button being offered. I'm coming at this from my experience, and I know it wouldn't work for me, but it could very well work for someone else.
Besides, someone has to play devil's advocate, right?