For a couple of decades I was loyal to NWA. The benefits of loyalty to DL are too much of a moving target, but currently I think:
- Upgrades are few and far between for PM (and even some DM routes) and almost non-existent for GM. $0
- Seat selection back in Y may be most worthwhile benefit left. $25 dom, $100 Int'l (EC worth a bit more)
- Board early to get bin space? $25
- Access to Hibbing elite line (when you can get them). $25
- Elite security lines aren't worth it because TSA makes sure they go slower than non-elite security lines (as they laugh at the lemmings in the elite line who won't even check out the other speedier checkpoints - because they are after all, elites). $5 (in case the line actually is better somewhere)
- Reduced luggage (and other) fees get calc'd in to the cost of the fare when I'm comparing. $x
- Various other benefits (SDC, non-Hibbing elite line services, etc.) $10
This is assuming all else fairly equal (similar flight plans, number of stops, flight times to my liking, where layovers are, etc.). On Int'l I'll pay more to connect in AMS or FRA than LHR or CDG, in U.S. I'll pay more to avoid ATL and JFK.