[QUOTE=J.Edward;8825978DEN seems to do well for UA and I would suspect the catchment area of DEN is impressive too.

As for a connecting hub...not sure. Maybe for regional markets but I would think DEN to be too far east to be a player on North/South West Coast traffic.
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As usual, J Edward provides a methodical analysis Just to add on his above comment, UA uses DEN as a relief valve for connecting east-west traffic because (1) ORD has pretty much reached capacity, and (2) seats out of ORD can be sold more lucratively as O&D for the large corporate travel base in Chicago. I read an article a while back about UA increasing DEN flights to connect more east-west flights.

CO hasn't reached saturation at CLE and IAH hubs for connecting east-west traffic and probably wouldn't benefit from another hub for that purpose. AA bought the failing TWA for an additional mid-continent hub at STL but didn't need it after 9/11. STL has even less O&D traffic than DEN, so AA has had difficulty making it a viable hub.