Oh, don't get me wrong, you can definitely read plugs and get useful information from them. Its just that any deposits that you can actually see and make any sense out of take a little time to build up. Reading plugs gives you broad information, not pinpoint accuracy for a short lived event.
I've gotta agree that dyno runs are expensive. My Harley has a Dynojet tuner in it and I had a coupon for MAPing for the bike. They came to a close-by town for a week-long annual bike rally they have there (Mountainfest; Morgantown, WV; late July) and I had it done while there on their traveling dyno. Otherwise I believe the local Harley shop gets $300 to do the same on their dyno. It is nerve-wracking to be there while its being done. They run the living dog-$#!T out of it until it just about makes you want to scream STOP! I couldn't even stand to be there while they were doing other guy's bikes.