The question is how long you can go without maintenance and repair--what's the cost over time?
It's like when you hit someone's parked car and they make your insurance pay to fix their fucked-up door, but the door had already been smashed in by them hitting a fire hydrant 2 years prior. How much more damage did you really do? Well, okay, a lot. How much more cost did you add to the repair? None. Why should you have to pay for it? Largely, because you're a shitty driver.
It doesn't make sense to me to claim that drivers of big vehicles causing big damage to roads should be proportionally more responsible for the damage they cause, rather than the usage they make, when much of the damage is unmitigated wear and tear--when the road takes its greatest damage from freeze-thaw cycles. If 10% of the damage is caused by vehicle traffic--that is, if the amortized cost-per-year is only 90% as much with no traffic as it is with traffic--then 10% of the cost should be scaled based on traffic damage, and the other 90% is most fair scaled to bulk usage.
Of course, scaling for bulk usage is stupid, too. It makes the percentage of income paid toward road maintenance higher for lower-income users.