I for know cables matter but we too can measure that. Talk to any layman about the myth of electrical components some how changing after 200-800 hours of use and they think you are retarded. Dig a little deeper and why would caps somehow start to change after 200 hours after what, a few million charge cycles? Also, why is burnin always good, why does it never make anything sound bad?
Now I for one know it takes a while to know any piece of gear. This time gets shorter with experience much the same way any professional who has to use his subjective skills does. I could really hold a lecture about bread and cake tasting and just how wrong and fucking clueless most people are. That said, the people who bang on the most about burn-in are the least experienced and have never held a soldering iron in their life. HAve no clue what makes the equipment tick nor how these supposed changes would come about.