I remain loyal to Delta because:
1 - They have the best operations on the LGA->ORD route which is where I do 80% of my travel.
2 - Stellar customer service willing to work with you when issues arise (ymmv, but i'm still turned off by the time I had a ticket on AA RUT->BOS->MIA where the RUT->BOS leg was cancelled and they could only fly me out 2 days later, when I ultimately decided to just drive to Boston they charged me $125 for the privilege of taking one less flight).
3 - Skypriority. As someone who is ineligible for pre-check, being able to get to the airport 45 minutes in advance and quickly check my bag and go through security is a huge advantage for me.
Is it worth an extra $200 on a short flight, probably not. Is it worth paying 10-15% more to fly Delta over AA or Jetblue, absolutely.
As an anecdote, am flying LGA->PTY on a first class award ticket in March. Most of our travel group is flying UA nonstop EWR->PTY for $475. One Delta 'loyalist' in the crew is paying $850 to take Delta with a stop in Atlanta. To me, thats absurd. He's GM, but $375 and a layover is not worth the 4000MQM and possible upgrade.