+1. For the distributor (of any type) to vary the timing by engine load or cylinder filling (and therefore charge burn rate) - and not just RPM - there must be at least one additional input. Manifold pressure (vacuum), intake air flow, and throttle position are common secondary inputs. In most applications - including most racing applications in the last 25+ years - the ignition has at least one secondary input to alter timing for best engine efficiency and therefore power.
The only exceptions are "on/off" applications with fixed load or fixed throttle, such as engine-driven electrical generators, irrigation pumps, drag race-only engines, pressure sprayers, small lawnmowers, etc. Any engine that uses variable throttle or sees variable load would benefit from variable timing with a secondary input.