This is true, when you're looking at flexy skis and using them on rails, any ski will break, doesn't matter what kind. He he he, and then these kids all whine about how their skis were obviously defective and how so-and-so's warranty is shit because they won't replace them.
Seriously though, I don't think that the durability or performance is inherent in the cap or sidewall construction. Look at it this way, if you are going to make a ski, you have planned out its design, its performance, its marketing, everything down to the last detail, even its lifespan. So, if you are planning on making a ski that is high performance and super durable, you are looking at an expensive ski anyway, so why not go ahead and make it sidewall, plus, then you can use that as a selling point too. Thus, most durable skis are going to be sidewall, but they aren't durable because of the sidewall. They were going to be a high quality ski anyway and if they had been made with a cap they would have performed quite the same and been just as durable. This is why you do occasionally see a super performing, super durable cap ski like the Prophet. If the sidewall was inherently more durable, then you wouldn't see the occasional fall-apart sidewall like the Liberties from a couple years ago and the first couple generations of AR5's.