I suspect that the convenience of a non-stop flight in a smaller market would probably trump any perceived advantage in comfort on a widebody - especially since a substantial number of passengers couldn't tell the difference between a 737 and a 747 to save their lives.
If you're flying BOS-MAN, for example (or even MHT-MAN
), why add two to three hours of travel time to your journey by flying through JFK, EWR, PHL, LHR, or LGW? And why deal with the hassle of flying through congested hub airports if you can avoid it? It's no pleasure waiting with 300-400 other passengers trying to find your bags on a luggage carousel, or having to board an hour before scheduled departure simply because it takes that long to get 400 people onboard.
BOS-LHR/LGW is a popular enough route to justify multiple daily widebodies. But there are some smaller markets in the Northeast U.S./Atlantic Canada which would benefit from 737 nonstops to the U.K. and/or Ireland. I believe there will be A319 service to STN from a couple of markets in Atlantic Canada this summer.