I am not crusading against stretch per se. What I am referring to though is shock loading or dynamic loading. The American Merchant Seaman's Manual refers to this in chapter one in discussing line in general. The working load of line is not applicable when it is shock loaded. Think karate and cement blocks! When a stretchy line such as nylon is parted the energy imparted to it in the stretching is instantly released in the opposite direction. A mooring line parting on a ship will leave the lines imprint in a quarter inch steel bulkhead, or cut a man in two. Dacron is superior to nylon in this regard as it stretches 60% as much as nylon and is 90% as strong. Chevron USA switched all mooring lines from nylon to dacron for this reason. Most ships use a combination of mooring lines and mooring wires. The synthetic makes for good spring lines and the wires, which stretch only 1%, are better breast lines, head and stern lines.
The point to having your lines tight is that you do not start the vessel moving. Any stretch that is employed in this goal is useful. The problem comes in once the vessel is moving. Now you have the weight of the vessel in play. I was tied up in Bayonne, New Jersey in high wind against a dock with rubber fenders. The vessel came a foot off the dock and, as the gust subsided, returned alongside the dock. The rubber fenders compressed, and a shackle on one of them, previously not in contact with the hull, punched a hole in the hull. Eventually, tugs were called in to keep her alongside. Anotherwords, the working in and out on the mooring lines was going to slowly beat the hull to death. The problem was that we were light (not laden) and there was no good lead available (horizontal) for some good breast lines. On a sailboat in that situation, when you leave the side of the dock your fenders may shift or even fly up and when the boat surges back along side damage will occur. As the oscilation continues you are going to start shock loading the lines and that is where you will start to lose deck cleats and part lines. Most boats are woefully under equipped with cleats for mooring and often have multiple lines to one cleat exponentially increasing the demand on one fixture. I haven't come across too many docking plans for sailboats, something the naval archetect does for all merchant vessels. The safest way to moor any vessel is snugly alongside with, in the case of sailboats, the use of good fenders and the often overlooked fender boards. Fortunately, for most sailboats, it's possible to get good leads on bow, stern and spring lines but those alone will not keep her alongside, hence those pesky breast lines with all their attendant adjusting. The original poster could probably get by with the 5/8" lines as long as he kept the boat alongside and doubled up for weather. He would have more "stretch" with those lines, right up to the point where he beat his hull to death on the dock or the line parted. And it would be the weight of his boat and it's inertia moving about that would cause the problem. In short, stretch is fine until the vessel starts moving relative to the dock and then it CAN become an enemy.
Regarding Casey et al, I find not too much to quibble with except that I have found double-braid, or 2 in 1, to be more subject to chafing, when used in mooring, than laid line. Dacron is significantly more expensive than nylon. And laid line, regardless of material, will stretch more than 2 in 1.
Another inadequacy is the average fender. Most are just filled with air and do a fine job of protecting your hull from scuff marks from aluminum row boats coming alongside. A celled fender, such as a Yokohama float, is much less compressible if not lighter or easier to handle. That's what is used when 'lightering' between tankers offshore. You bring the two ships alongside with the floats between the two and tie up securely. The crew adjust the lines as the smaller tanker sinks to her marks. The fenders don't blow out because of their closed cell construction. Other than collapsability, the average yacht fender is, in my opinion, over-priced junk. You could do as good a job with those foam tubes the kids play with in the pool.
On a somewhat related note: I was tied up in Dutch Harbor to an oil platform loading crude. 70,000 ton tanker deadweight with 18 parts out. It was in January and when the tide turns in the Cook Inlet all the ice comes flowing back to the south from what seems like all the way to Ancorage. The angle of the platform is such that it catches you on the stern quarter. Finally had to stop loading and disconnect the cargo hoses. Good thing too as we parted all 18 and that was with the engine going dead slow astern. Sounded sort of like 18 syncronized shot-gun blasts! Once the ship started moving it was over. No one on deck, or injured, but we junked about a hundred thou. in mooring lines in about 30 seconds!