The shape of the power curve can give a lot of insight as to what application the engine will be most useful.  An engine that has a broad flat torque curve will provide good acceleration and be easy to ride when traction may be a factor.  An engine with a high average power level over an RPM range that is defined by the transmission will be easy to keep in the power range. 

Power and torque curves that have spikes and dips are indications that components (pipes, ports, reeds, ignition time curve, intake tract length, or air box resonance) that comprise the built are mismatched or the pipe needs more development.  Engines with smooth torque and power curves are easy to tune.  An engine with dips and spikes in it’s curve may be impossible to tune because carburetors do not have the ability to make sudden changes to the mixture at a particular RPM at wide open throttle.

Most of the professional racers I have worked with have trouble distinguishing power changes that are smaller than 3%.  The stopwatch or time slips will often indicate the affect of small gains or losses but the riders butt dyno will not.  Most recreational riders butt dyno needs at least a 10% power or torque change for it to be noticed.  Using a dyno for engine and pipe development is a time saving device and usually results in a more reliable and higher power level when used with adequate racetrack testing. 

The majority of the changes made to engine components during high-level engine development will have affects on the power curve that are less than 2%.  The gain and losses are so small that doing engine development without a dyno and using only track testing makes it very difficult to distinguish the small changes and keep all of the small positive gains and omit all of the small losses.