Doug, I think there were some pretty direct and well directed answers concerning the OP's question. The boat he mentions (after his initial post) has in mast furling, which needs the additional structure offered by double spreaders for stiffness. Let's take a step back. Let's assume a boat has a very standard slab reefing system, so there's no furling system variables. I'm not a voice of any authority, but here's my take assuming an aluminum mast:
50's-early 70's, single spreaders were the standard. The load analysis tools that were available in the day to the average spar maker, even aerospace, were pretty crude by today's standards. Milling and extrusion tech was good, but there was significant costs to engineering thin wall sectioned structures if the project wasn't intended to literally fly. So there we go, thick mast sections and a single spreader where pretty bomb proof and relatively easily manufactured. We're also talking about wire vs. rod rigging, so there's a bit more movement to consider as well. I'm still blown away when I see an old Nautor rig; a tree trunk section w/ check stays, baby stays, the whole thing.
The downside? For cruising, not much, but for racing, it was weight aloft and the inability to change foil (sail) shapes, and much wider genoa sheeting angles. Next came the inline double spreader rigs. Mast sections got much smaller and lighter. Add running backs and check stays, even lighter and smaller as rig designers squeezed the section higher up the rig. The rigs were relatively unforgiving, but for the owners of race yachts, it was worth the performance gains even if measured in inches and boat lengths. Eventually this stuff becomes refined enough that it gets adopted for cruising applications, in this case sans the more extreme mast sections. Smaller sections, less weight aloft, changes in what goes under the boat (think back to how 'radical' the Valiant 40 was thought to be in the age of Westsails ) At this point, race boats are now into triple spreader rigs and even smaller sections. Now the masts are very bendy (rod rigging and low stretch mylar/kevlar laminate sails start appear... loads increase while rig weight continues to decrease ) giving the added benefit of more sail shape control. This coincides with the arrival and acceptance of ULDB boats (Ballenger Spars inlineljjkAll this rig & sail technology continues to make it's way into the cruising world, just not to the extremes of what we see on race boats. So now, many cruising boats come equipped with swept back double spreader rigs (eliminates the need for checks & runners, but has one disadvantage of wear and tear on the main as there's more contact with the spreaders at deeper sailing angles)... etc, etc.... In the mid 90's, carbon rigs/hulls make regular appearances on race boats. Mast sections continue to get smaller/bendier, and sail technology begins to move from paneled sails to a single surfaced shaped sail a la something like North 3DL and UK Tape drive... this is also now widely accepted in the cruising community for those with the desire and budget... Yesterday's blue water bombproof boat is still a great boat in it's simplicity, ease of repair and maintenance, but much slower and heavier than a more current off shore design. Are there trade offs? Sure. One that particularly sticks in my mind is an older S&S Swan that had a new double spreader rig installed... the weight aloft was greatly reduced and the boat sailed much better at the top end of a given wind range before having to reef or change/shorten headsails.... anyway, out of time, etc....
Mitiempo, is that "Heart of Gold"? Your first image looks like the rig from one of my regular race rides.