Mark, very well put. I invested in a Dyno jet chassis dyno about 4 years ago. Ive raced and built shifters for 18 years. My clients and myself won many times without the use of a dyno.
With that said, I purchased the dyno when I jumped into a 250 superkart. With those karts you cant just go to the local track for testing. The dyno became an invaluable tool for me. I built a mule chassis so I could dyno the engines I built for customers. When I handed them a fresh engine with a dyno sheet, it increasded the confidence of both myself and them. So when they got to the track, if the kart would not fall out of a tree, they knew to change there set up and or their driving. That eneded the your engine doesnt make any power problem.
Another customer I convinced that the dyno was worth it is a gentlemen that races several LO206 class karts. This is a sealed engine. But we found 1 hp and solved his problem of the kart laying down at te end of a long straight.
Is a dyno completely necessary, no, but it can provide a lot of info on what works and what doesnt. All the engines and karts that leave my shop have been dynoed.