For me it's usually a balance of result needed, costs, and availability - in that order. Shelf grinds are good over wider rpms and different types of builds fr the most part. They also tend to be less aggressive which for some might be bad, but for me I like longevity and don't seem to have any trouble making power with them. So while i do use custom cams I can usually find one that will do what I want and once I find one and have that result, I'll probably use it again.