On the passagemakers having singles thing, that doesn't bother me as much as a single in a coastal cruiser. Losing propulsion when you're 100+ miles from anything you could hit isn't an immediately critical situation unless you're in bad weather. You've got time to figure out what's going on, let things cool a little, make some repairs, etc.
In closer to shore with much less sea room, more traffic, etc. then sudden loss of power could be a bigger issue, so being able to retain maneuverability during the event can make life much, much easier.
Thinking about it, having twins may have saved me from needing to dive last year. I sucked up some weeds (in both engine intakes) somewhere in the Erie Canal. Didn't notice right away, as things were still cooling fine at the 1300 rpm I was turning for most of that day. But as soon as I asked for a little more power, both engines started to heat up a bit. So I shut one down, ran on one engine for a few minutes to let some water flow over the other intake (scoop type strainer intakes), then switched engines. After this, flow was better, but not quite normal. So I gave both a blast of hard reverse, then repeated the exercise and then cleaned out stuff that had been dislodged into the strainers. After that, cooling was back to normal.
Twins do gain redundancy on items beyond the basic engine itself too. On a single, alternator failure means you're now relying on the generator (if equipped) for 12v power, as the engine isn't producing any. With a twin, you've lost 50% of your 12v power source (and can still start the generator if needed), but you still have power. Depending on where you are, what parts you have on hand and when / how easily you can get parts, that can make a big difference.
Same story if you ding a prop on something significant. Assuming you're not equipped for an on-the-spot switch to the spare(s), the single leaves you limping very slowly or being towed to the closest possible spot to get it taken care of. The twin would let you limp along on your own terms for repair.
Of course, as others have pointed out, in many cases, the boat dictates the choice of twin or single. And even if you have the choice, sometimes the equipment layout or choice of engines, etc. makes the choice obvious regardless of which you'd prefer. And as Willy points out, it's rare that builders really optimized when going to twins and they often just stuck a second of the same in there instead of going with 2 smaller engines (assuming the single was already well powered).
Along those same lines, there are some cases where a single just wouldn't make sense. In most faster boats, a single would start to become very large to get enough power. My boat, for example, has 2x 340hp engines. So for similar performance from a single, you'd be looking at something close to a 700hp engine. That would force the boat to be diesel as a start (no available gas engines with adequate durability at that output). But the bigger issue would be fitting that monster in the engine room. It would likely be an issue height-wise. And with it being that big, you'd be blocking a lot of the main bilge area, making it harder to clean, harder to access pumps, etc. So while engine access may get a little better, overall systems access likely wouldn't. You'd just have more dead space out at the sides of the engine room.
:lol: 
