I think it is a mistake to think that the Skier's Responsibility Code is about assigning blame. The point of the Code is to show people some basic ways to ride responsibly to minimize the inherent dangers in the sport. While it provides a good high level view of safe riding, it is not as if it provides guidance that is both specific enough and comprehensive enough where we could say, "gee if only everyone adhered to the Skier's Responsibility Code no collisions would ever occur." For all but the simplest of situations (such as the knuckle-headed tree skiers in the other post), the code isn't all that helpful (as MR's post points out).
To me, the more interesting question is less about who is to be blamed and more about what can we learn from these situations to help us become safer riders. Two things stand out from UL's example. First, if you are going to be making a traverse across the hill, you have to be cogniscent of riders above you so that you don't cut them off. Second, riders who are downhill of you may be capable of covering lateral distances much faster than you might think.
To me the latter point is what makes the Skier's Responsibility Code somewhat irrellevant when it comes to dishing out blame. Here's an even less cut and dry example. I was coming down Haywood at Keystone, skiing skier's left of the center of the trail. There was nobody else on the trail with me. I was probably 15 feet from the trees on the left, making short turns. Conditions were icy and I was on race skis. As I reached the bottom of the first pitch, I decided to switch to GS arcs and made a left turn towards the trees while simultaneously dialing up the speed. Turns out, a ski patroller (on a snowboard no less) had decided to pass me on the left (despite the fact that entire trail to the right of me was wide open). Fortunately, I picked him up out of the corner of my eye and reversed my direction and avoided the collision, but it required a quick reaction. So is one of us in the wrong here? Despite the fact that he yelled at me to slow down (which was odd, since he was blowing by me at the time--trying to catch his buddy who was sledding down some old carpets), I don't think it was me. And other than the fact that he was riding too fast for his ability given his equipment and the conditions, I wouldn't necessarily say the boarder was at fault. Certainly, a much better choice would have been to pass me on the right where there was plenty of room, but my skiing right up until the point where I switched to GS turns was telling his brain that he had plenty of room on the side he chose. My ability to make large lateral movements in a short amount of time was unknown until I did it, so it didn't factor into his decision making process.
Truthfully, I don't think those types of collisions are completely avoidable. Had we collided, it would be very easy to say in hindsight that he should have given me more room, but I'm sure he thought he was giving me enough space to begin with. I could have looked over my shoulder as I widened my arc, but my path was going to take me close to the trees, and given the conditions, my priority was all about making a clean turn. The Skier's Responsibilty code would put the blame on the boarder for not giving me enough room to make whatever line adjustment I wanted (or needed), but that wouldn't have been much consolation if we had collided. If the situation would have been reversed, I would have been thinking "I gave the guy more room than most riders give each other on the hill yet we still ended up in a heap."
What is scary is that despite trying to be proactively defensive, I still have a few near misses every year. Just the other day at A-Basin, I was coming down Dercum Gulch to the Black Mountain Lodge, skiing quickly, and there was another skier on the far left side of the trail. I immediately moved as far over to the right as I could get and started watching the guy, to make sure he didn't become a factor. As we neared the lodge, he swung left and I mentally dismissed him since I thought he was heading all of the way down. Instead, as best I can figure, it turned out that he was just swinging around a group of people and as he cut back in, we almost collided. He was completely out of my line of sight, and then back in, just like that. I don't even know who was technically the rider ahead at that point; all of the sudden we were both on converging paths. Again, quick reactions prevented the collision, but I'm left scratching my head about how the near miss could have been avoided. What I could have done would to have been to slow down and let him go ahead, but I wasn't thinking that at the time--despite being mentally on yellow watching the guy.
In any event, I find it disturbing that this stuff still happens even when I'm doing everything I can to give people space, look around, and ski defensively. Perhaps, as TPJ says, a miss is as good as a mile, but it doesn't feel that way when it happens.