I agree with what others have written, the rollers can wear out over time, and it doesn't matter the kind of track. The rollers are sprung so differences in elevation in the center rail shouldn't matter (in theory shouldn't happen, but hypothetically if a center rail warps up or some such). Rollers might wear more if the spring is very stiff but in reality I suspect that the wear happens relatively evenly over time. Rollers that are softer will wear out faster than those that are harder, and given the history of Lionel with the changes in owners and production, wouldn't surprise me if the rollers had variable quality *shrug*.
Carl's post is also a great observation, a lot of people likely don't routinely oil the rollers and/or clear them to make sure they are free rolling, so what happens is the roller in a sense can become partially a sliding shoe, if the roller cannot turn well then there will be wear caused by the roller not turning as fast as it should,pure and simple (kind of like if the front wheels on a car don't turn and wear out the tread when you move forward).
