I have never experienced an actual buddy running out of gas, either. The closest I came to it was when I was in a group of diver on the Spiegel Grove in Florida. One of them had apparently not noticed she had put her gear on a near-empty tank, for she went out of air very early in the dive. When that happened, she very calmly swam over to her buddy and took the alternate from the keeper without giving a signal. We all then ascended together.
I was then working in a shop that had about a dozen instructors, and I asked them to recount their experiences. Only a few had had experiences with real OOA situations, and every one of those experiences was exactly as I just described--the OOA diver calmly went for the alternate.
OK, I started another thread to collect some firsthand experience. Interesting question!