I think another issue with mechanization in COIN is the basing and logistics requirements it brings. Basing equals a larger "foreign" presence, which in turn creates an issue that an insurgent force can exploit. Since mechanized forces consume and need more in the way of supplies, they require a more extensive support network than light units, in turn creating insurgent-exploitable issues and targets.

You make a good point regarding the possible causal impact of mechanization; one that I think the original paper missed. As for the million-dollar question...it may come down to having units/forces that are trained/optimized for COIN and others that deal with larger conflict issues (which as I recall was one of the original arguments for the light divisions). Mechanized units have a clear place and utility in COIN, but their role is different than what you would see in a more conventional conflict. That might require different training and equipment (a good role for the traditional-style dragoon force slapout mentioned...or the traditional function of US cavalry prior to the world wars). Usually it's a question of force scale, and sadly our institutional "either/or" mindset hinders our ability to find that balance.