To the best of my knowledge (correct me if I am wrong)

Routing rules are strict on CO. LGA/EWR are not co-terminals. You can't decide to go to LGA when your ticket is to EWR. Execeptions are made in bad weather/irrops - also depends on the agent. trAAsh is more flexible with LGA/JFK/EWR co terimnal switching for customer recovery, CO enforces routings like it's gospel. At the end of the day, my home is in New York, and I just want to get home.

trAAsh allows you to add cities and be creative with city pairs, up to the maximum permitted mileage, and make all kinds of funky stops along the way. I believe this is a sabre thing.

CO on the other hand has mandatory routings, i.e you are going from EWR to DFW, you must fly either EWR-IAH/CLE-DFW, You cannot get creative and fly EWR-LAX-IAH-DFW on the same fare basis - it prices out completely different and becomes a fare to LAX and DFW.

Also, there are markets where CO just chooses not to compete. i.e LAX-HKG.The route for that would be LAX-EWR-HKG, a fare on OZ and UA (Star Alliance) can be had for less money, direct across the Pacific.

That being said, there are some bargains, but in general, (and I am sure I am not alone here) fares have been redicioulously expensive and the airplanes have been full to the brink.

What's the alternative? . Take a train across the country? We need to fly. (at least I do).