What need to happen for you to have to buddy breathe? You have to be out of gas, and your buddy has to have one regulator fail. The likelihood of those things both occurring on one dive is extraordinarily low. For the "active" diver (who dives ten times a year) I think it's vanishingly unlikely, and I have no problem with BB not being taught.
In addition, for someone who dives enough to develop some in-water skill, BB is actually pretty easy to figure out for oneself. My husband had me try it with him a while back, and it was pretty trivial.
On the other hand, for folks who are going where the surface is not an option, the composure it requires is probably a reasonable thing to practice.