I had an excellent opportunity on this last trip to compare the virtues of a rolling suitcase with a convertible backpack. The backpack won, hands down.
Despite knowing my philosophy about rollers, my partner, Robin, insisted on using her 20" rollaboard on this trip. Due to her health problems that led to her being in the hospital and rehab for about 3 months earlier in the year, she is really not strong enough to handle any luggage by herself. I had to handle both my backpack and her roller. It actually worked out well, because I would have had trouble with two backpacks. But it sure brought home the difference between a backpack and a roller.
Changing trains: As we approached the station, I stood up and put on my backpack. Then I picked up her roller and carried it up or down stairs to the door level (a lot of the trains we rode were double-deck with the door at an intermediate level. I picked up the roller to take it off the train because the platform was almost never in line with the train level. Then I opened the handle and rolled it to the stairs. (Note: in a few stations there were elevators to the tunnel, but in most cases I had to use the stairs). At about half the stations, there was a conveyor belt for the bags to or from the tunnel, about half the time they were out of order, so most of the time I carried the bag down to the tunnel. I opened the handle to drag the bag to the next stairs, then folded it and carried the bag to the platform, then rolled it to where the train would stop. When the train arrived, I folded the handle and lifted it onto the train, then up or down the stairs in the train to our seats. All the time I was fussing with the roller, my convertible was there on my back, requiring no attention.
Getting to the hotel: From the train to the station, I had to lift/roll the suitcase as I described above. Only once, in Rothenburg, was the station at street level and close enough to just roll the bag. A couple of times we had to take a taxi. At the hotel there was always stairs from the street into the hotel (lift again). At only one hotel was our room on the ground floor. At the rest it was at least one floor up - carry the bag again. Again, all the time I was fussing with the roller, my convertible was there on my back, requiring no attention.
I should mention that my normally 13# backpack was 17# because I was also carrying the charger for Robin's oxygen concentrater and the extra battery for it - another 4# - but my backpack was still far, far easier to handle than her rollaboard.
So, OK, we can dismiss the myth that rollers are a problem on cobblestones, I never had any problem, but all my "travel" was on paved, or cobblestone, roads. Try pulling a roller 4/10 of a mile up the dirt path from downtown Bacharach to the hostel at Stahleck. But we can also dismiss the myth that you never, or rarely, have to carry a roller. I was carrying it all the time.