too burdensome
Indeed, 'tis a trade-off. Each time/effort/cost break-point is different.
Considering phones have cameras and I am walking around a car looking for dents anyway, the time+effort is well below the cost of an extra 1-2 minutes. Add in the negative and high cost (though low percentage risk) of a bad rental company and the break-point makes sense to me. But, to each his own