Kev - you don't have to "order" a custom like you do a regular cam. The designers will request a bunch of information from you about your set up (head, intake, exhaust - flow data, compression ratio, stroke and rod length, etc.) and also ask you to answer a lot of questions about what you're after, how you use the car, if it has to pass emissions or not, what kind of idle you can live with, where you want your peak power, etc. -- then THEY model your set up and THEY determine the specs of the cam. That's what makes it a custom.
An off the shelf cam can perform just as well as a custom - IF, big if, IF you happen to guess correctly and pick a cam that works the way you hope it will. I'm not willing to leave that to guessing and hoping - hence the custom route. With the model, the guessing/hoping is pretty much gone.