Back to assigned seating.
If all the stations along the route could be converted to high-platform (fat chance, I know) or the rolling stock converted to some sort of bilevel with level boarding at low platforms, then I speculate that assigned seating would be a thing of the past. As it is, they seem to try to sort people out into cars based on what station they are detraining at, so that they only have to open one, two, or three doors at any given low-platform station.
As it is, I wish that, rather than having your seats be completely up to the conductor, it could be incorporated into Amtrak's reservations system. Say you are traveling CYN-WAS. The reservation system then "knows" that only the 4th car will be opening there, so it restricts your seat selection to anywhere in that car, but allows YOU to be the one to choose. The policy of keeping large blocks of seats reserved for large groups is a nice thought, but the best seats in the train really should be first-come, first-serve: ie, you book first, you get first choice, as on most airplanes.
I have definitely boarded at CYN for a trip to DC, and had friends board 10 minutes later at RGH, but be given the ultimate "evil eye" by the conductor when asking if I could be moved to an open seat closer to my friends.