Standard practices became standard practices because they worked. Virtually all the standard OP battle practices, that we have seen, are indirect. When you use something that you "know" works, you often do not realise when it fails.

C armor would also not nesecarrily be apparent, so channelers would not immediately know they needed to go to more thoughtful tactics (such as blades of air through the eyeslits of a helm). In fact, the average channeler would probably assume that it is just a bunch of White Cloaks and thus without any OP defenses.

If you are used to dispatching a knot of men by waving your hand and sending a giant blade of air at chest height, when it does not work, you may well end up dead before you figure out why.