Smaller jets at mainline means more jets at mainline. That means the pilots already there will enjoy more seniority, more basing options, etc. It also means more captain positions... and those small jet captain positions probably pay better than those MD-80 F/O positions.

Plus it allows the airline to operate more efficiently instead of dealing with some outsourced carrier and the associated overhead costs. And it gives you more control of your product... Delta wants to give a good product right? They sure aren't doing it with a lot of those RJ operators.

There is absolutely no reason to not want any particular plane on the mainline list and anyone that tells you otherwise probably has an ulterior selfish motive.