The problem with Street Touring (ST) classes is that they limit the locals from modifying their cars to their taste and are blockaded in a certain ruleset for modifying their cars. You usually cannot modify body panels (body kits) or do any motor modifications (only bolt-ons) so that's why our ST_ local classes are limited. I think the people that filter into the local Tire PAX are the ones in Street Prepared and Stock classes that don't want to run r-compound tires.
You're right. I see the logic of going to Tire Class if your at the Street Prepared Classes or higher (regarding number of mods).
I guess I'm really speaking to the Stock class folks who don't want to run with r-comp tires. It makes sense to me for them to consider running in their Street Touring Class where they would run on street tires and the number of mods they would run against are considerably less than they're likely to run against in the local Tire Class with SP, Prepared, Modified cars.