Some of these arguments made more sense back in the day (likely when they experienced this) when Part 135s and mainly prop/piston aircraft were doing actual regional flying, feeding mainline.
Things couldn't be more different than that now, with "regional" jet aircraft more modern and better-equipped than many, if not most, "mainline" jets. And now, these airlines have to meet the same 121 standards and fly routes that are even longer than many of the "mainline routes". Things made a paradigm shift a few decades ago, but they (the airlines) have managed to keep some pilots on a "B" schedule, despite having the "regional" flying rise to the level of "mainline", to make their bottom line better, by pitting these smaller "regional" airlines against each other. Like it or not, "regional" pilots are flying jets that fly just as fast as mainline to the exact same places, it ain't 1970 no more.
From a safety point of view, the goal should be just as much experience and safety at the "regional" airline level, as compared to "mainline".