I think dyno's are a great tool, but nothing more. It's merely another method of gathering information. There's a LOT of information gathered before an engine build ever even begins and the dyno is just a continuation of that information gathering process. It's also a great place to baseline tune an engine in a controlled environment, especially a new or unknown combination. It's very helpful to be able to go to the track or wherever knowing basically where you stand, and what to expect. You might also realize on the dyno that going to the track will be a waste of time and money (and could prove real embarassing), and you head back to the drawing board.