I use 20% charcoal in my mix. I don't have a scientific reason for it. I just like charcoal. It absorbs a variety of nasty substances, which is why it is known as a "sweetener" (from its absorption of acid, presumably). Does it benefit the plant? Can't say for sure. It doesn't hurt. And the 20% doesn't significantly affect capillary action, in my opinion.
Charcoal does not break down, although I suspect its ability to absorb various compounds diminishes over time. It is also prone to retaining salts, but that usually is not a problem with s/h.
Bottom line: I don't think it makes a difference if you use charcoal or not. Until someone proves that it has some quantitative effect on plant growth, I believe it is more of a personal preference.