As far as I am concerned if you're good enough to fly Delta passengers on a CRJ for PNCL, you are good enough to do it in a Dal 737 etc.......
The un-hireable personalities included? Remember, 9E has taken guys without even having any interviews. Straight from an approved college program into a 9E newhire class. There's a big difference between RJ flying and mainline flying. On a RJ, you can hire a bad personality and not have to put up with it for more than an hour to two hours. At Delta, you have 737, 75/76, 777, etc, and have to fly with guys for 7-12+ hours. Here, you need to make sure you weed out as best as possible all the bad personalities.