The reason (according to Poliquin and others) is that some small muscle groups get too much stimulation and are therefore in a constant state of being overtrained/unable to recover. With Biceps being so involved in any kind of pulling movement (even deadlifts, to a certain extent), training them directly in addition to everything else (back work and so on) can easily result in overtraining. Especially when you throw in the fact that most newbies make the mistake of over-training arms to begin with.
My arms grow easily compared to everything else, and I’ve been trying this idea of CP’s in my workouts for over a year now. I haven’t had anything but positive results. When I used to work arms a lot directly, they were about 14 1/2 cold. Now, with only very occasional direct work, they’re closer to 16 inches. Proof of the pudding, at least in my case.
You’ve mentioned several times that you’re more endo than ecto (whereas I’m the opposite), and therefore you probably don’t have to worry about overall recovery as much as someone like me does. Also, arms may not be an easy bodypart for you. But for someone in my situation, not directly training arms has been great.