Most ships do not carry cleats for one. Bitts, while far larger, are much eaier to use and make for a much more secure line. Research has been done on securing synthetic lines to bitts and, due to their tremendous amount of contact area with the line, it's highly recommended that two full round turns around both heads of the bitt be taken before finishing off with figure eights. No locking turn is ever taken or needed for that matter (all it can do is cause trouble later). The round turns have less contact with the two bitts and therefore produce less friction as the line works on the bitts. Where this really manifests itself is upon releasing the line. If you've figure eighted the entire way you'll find that the line will have fused a small amount to the bitts. You have to remove almost all turns before the line will break loose, often resulting in loss of control of a loaded line and possible injury. With the two round turns you are able to remove the figure eights and easily surge the line while maintaining control. That the majority of the load while secured is taken by the two round turns also results in less damage to the line versus securing with figure eights only.
Given the relative smallness of cleats and bitts fitted to sailboats I see no real problem with a locking figure eight to cap off the line's securing. It adds no extra grip to the hitch but it does allow for deck traffic and wind that are more of a factor in the hitch coming adrift from the top down on a sailboat. if the locking turn is going to take a strain and bind up, making it difficult to release, I'd submit that the cleats are of inadequate size. Which just about describes most boats, right? (g)
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.