Why you say?
While you particially answered your own question...there is more too it. Tossing in a colder T-Stat without tuning the car for it will do nothing but hinder performance and MPG's and put more of a premature wear on internal parts when not warmed up. The reason to run a cooler t-stat is also for the sake of...and the reason I do it...since the motor is running a little cooler (and like 15-20* difference is going to destroy your motor anywho...lol hardly) your not getting all those parts that like to break, rot, and fail due to heat...example, coolant elbows, LIM gaskets, Upper Intake Manifolds (97-03 cars), vacuum lines, rubber "T" and "L" fittings...and other misc. electrical things.
And what is the firm beliefs on that? How else would you cool the motor itself down? Stock Thermostats are for stock cars. No mods...no intentions of going fast. If you see it a different way, I'm more than open to listen to ideas.