I believe they are useful to some people. The extra weight that can be pressed in the decline gets one used to pressing heavier weights. Although I've never had that problem so to me they are completely useless also.
I like this thread though to see what exercises stimulate areas more and such. I think it would be great if they had included the other muscles that are working and to what percentages. For example, what percentage the hamstrings, calves, and glutes worked at in a normal squat.