I will add that the foot has three arches to it that should be supported. Dropping of any of the three arches will lead to improper gait, therefor altering the normal biomechanics of the ankle, knees, pelvis and spine.
I will disagree with Kirk. In my opinion, we are on our feet much more than we intended to be. I don't think our body we were designed for that type of abuse. So I would say that proper support of the arches is critical. Custom orthotics are very useful at supporting the three arches and preventing problems. Why wait until they drop before supporting them? If supporting the arches were not important then why would the shoe manufactures attempt it in nearly every shoe made.
In my office, I have the equipment to scan the feet and look at the pressure pattern that the feet are producing. I have practiced for 8 years and have seen about 3000 new patients in that time, the vast majority have some sort of foot problems.
As far as the crocs being a good shoe... I don't know. But what is a good shoe for one person, is a bad shoe for another.