The formula I posted was strictly for blades mounted to the engine crankshaft. Most ZTR/riding machines have blades driven by pulleys, belts, chains, gears, etc. so the math would be different.
Any mower engineer will tell you a faster spinning blade will almost always provide a superior cut and finish quality. You are also correct the deck design and internal aerodynamics (airflow under the deck) are equally important. I'm not aware of any mower manufacturer that ignores the ANSI standard, and all must follow the Consumer Product Safety rules governing thrown objects, which is directly related to blade speed.
I think most consumers tend to make decisions based on any number they can use to make a comparison. Be it blade tip speed, cutting width, self-propel speed or horsepower, it is simpler to compare different products. If one ZTR runs 17,000, but another runs 18,000, the 18k must be better, right? Sort of depends on what you define as better and no doubt that's a whole other can o'worms. :confused2: