I'm not sure that I understand what you're asking. Do you mean that airlines are replacing their major "big city to big city" routes with the regional airliners, instead of just using them for the "little city to big city" routes. If that's what you mean, then I've noticed it, too. Anymore, it's hard to tell where Comair leaves off and Delta begins. If a major carrier like Delta has a close relationship with another smaller carrier like Comair, they can balance each other's schedules so that (on the routes that they share) they're essentially the same airline. For example, both Comair and Delta fly the GSO-CVG route. There are 6 flights a day (3 Comair, 3 Delta) and the departure times are perfectly staggered. They both fly jets, so there's essentially no difference between Comair and Delta. Comair does plenty of big city routes, like CVG-ORD, CVG-EWR, CVG-IAH, CVG-MCO, etc... It seems that it's not just a feeder airline for Delta anymore, but an integral arm of their mainline fleet.
The RJ's don't just have a place in the little airline fleets. Midway is getting them like crazy, and Air Canada loves to use them on low-capacity routes to America. We also just heard that LOT is getting some ERJs. I'm sure there are other examples of that trend.