(Edit for clarity) If you are diving around a bunch of strangers of unknown training and skill levels, you are being a better emergency buddy to them if you have a reg to donate, and if that reg is in one of the places they have likely been trained to look for it. (eg, snatch it from the PADI triangle, or you donate your primary and use a necklaced or inflator-integrated backup.)
You aren't obligated to think about how to be a better buddy to a pack of strangers, but it's the decent thing to do.
That was my point. But, some people seem to think that a regulator slung around ones neck on an extreamly long hose is the norm in the diving community.
Other than one diver (a SCUBA board regular) and SCUBA board, I have never seen the long hose set-up in use other than for deep cave penitration or wreck diving.
Back to the original question, IMHO (ooo I used a blogger acronym) the safe-second/octo is just a convenient method of sharing air to help a second diver to the surface. It is not a necessary item of equipment that can be replaced with a number of other techniques.