Good question–and, diplomatically speaking, you are both right. The mainsail does provide support to the spar. It reduces pumping action and when combined with pre-bend, nicely locks the spar into place. However, this is not an engineered function–but it is still indirectly in most rig safety factors.

As you get into cruising rigs, there are usually high-enough factors of safety to allow for the main to be dropped, and sail only with a headsail. As you have noted, there are lots of images of this in Surviving the Storm. Note: extra backstay tension to induce pre-bend, and carefully set runners all help reduce the tendency for the rig to pump or invert.

A couple of key points to keep in mind: When the main is reefed, the head of the sail exerts an aft component of load at wherever it falls on the spar. This is trying to invert the spar which could lead to a “gravity storm" (in layman’s terms, the mast falls down!). So, reefs should be engineered so the headboard occurs close to or preferably just above a forward support point (such as fractional headstay or cutter stay). If this is not the case, then make sure that extra backstay pressure is applied to induce additional forward bend to counteract the aft pull of the main.

Years ago, in an SORC race, there was some breeze and everybody had to reef. Those spars were huge by today’s standards. A bunch of them fell down in a really large gravity storm and there was a hue and cry about the skinny rigs the racers were using. After investgation they found that the problems were caused by deeply reefed mains opposed by large headsails, with no form of forward support on the masts anywhere near the head of the mainsails. (There is a bunch of data on this on Offshore Cruising Encyclopedia). Regards–Steve