This probably belongs on another thread, but, don't believe everything you hear on this board. The operational difficulties are gone. TWA currently has as more labor issues than US has (and so do other airlines like NW), and high CASM is based on having their bread and butter in the northeast where it is expensive to fly. BTW, they make a killing off of those expensive to operate flights because there is that much market for them, and they serve it very well. Forget the CASM, it's misleading.
Now, as for the 753. If given the choice to fly a 753 when everyone else on that route would be flying widebodies, you bet your gerbil I'll take the widebody anyday. Two aisles are just so much more comfortable when there are that many people on board. The 753 was designed for charters really, or airlines with no intentions of putting 767s in their fleet. Its kind of silly to put 753 in your fleet and still have 762s.
BTW, US does not fly 737-300s coast to coast. It's all 757-200 (a very remarkable plane) and 319/320. If you've flown them in recent years, you'd brag too.