Anyone who can recline on a bed of nails can do so only because there are enough nails present to evenly support their weight. Here's how it works. Imagine a 5'6", 120-pound man sleeping on a bed of nails; let us say that the width of his body averages out to 12 inches in width that need to be supported by nails. Imagine also that the man's skin actually comes in contact with only 4 nails/sq inch (the numbers I'm using here are strictly hypothetical. PLEASE don't try this at home!!). Hence, in terms of nail contact, the man is 66 inches tall and 12 inches wide, for a total of 792 square inches available for nail contact. At 4 nails/square inch, that makes for around 3168 nails to support the man's weight.
So when you multiply it all out, the actual weight supported by any given nail (at 4 nails/sq in) is really pretty small: 1.65 ounces/nail. Spread it out over 6 nails/sq in and the number drops to 1.1 ounces. That's not a whole lot of weight.
No, the bed of nails won't be comfortable, but it sure as hell won't do the damage that 120 lbs against 3 or 4 nails would do!
Why is this breakdown in an acoustic guitar buying guide?? My point is that 12-strings distribute the potential "cutting" surface of string-against-fingertip across a greater area. In my opinion, this can make the pain of playing less acute.