A couple things from reading above. Selvedge doesn't make the jeans better. It's simply using the selvedge line (meaning the very edge of the material when produced on a narrow, normally vintage loom) to make the outside seams of the denim. In order to make a pair of jeans show the selvedge, that seam has to be perfectly straight. To make that happen, you have to make the shaping come from other places in the jeans. When you have a jean fit like these (or an SLP d02), it's very hard to get that fit and kept the selvedge intact and straight down on the outside seam, which is why you rarerly, if ever, see it on a fit like this.
Also, 3x1 as compared to a 2x1 is simply referring to a twill pattern. Each woven has their own traits, which is why they are chosen or used (if the designer or fabric technician is very technical). However, a 3x1 twill isn't more valuable or more prized than a 2x1.
A lot of terms surrounding all these items are very misunderstood in here. There are many places that you can learn about these things and I'd encourage you to do so, before you blindly proclaim things.
Ok, I'm going back to actual work.
-I agree with you, we prob can't comment on quality of denim based on whether it is selvedge or non-selvedge. The quality of the denim is really dependent on numerous factors(including but not limited to):
1. Type of cotton that is used ( companies don't tend to comment on that unless it's something like supina cotton or organic cotton)
2. The way it was made (Open ended vs. ring spun vs. dual ring spun)
3. If it has been treated (mercerised, etc)
4. How it was dyed (rope dyed)
-With regards to Selvedge denim being better or not better, I think we can both agree that selvedge denim is thought to be more "durable" because the selvedge ID (or line as you said) protects the edge from fraying, unraveling, or curling. Here's a link: http://www.artofmanliness.com/2014/04/22/raw-selvedge-denim-introduction/
-I can't speak to intricacies or difficulties of making skinny jeans, i have't seen any articles about this. But I am interested, if you have any links that would be helpful. One thing, I would like to bring up is that the Cast 2 raw stretch and the Coast 2 have similar dimensions to the MIJ denim released yesterday, both of which were made using selvedge denim. So I think it would be feasible to have made yesterdays MIJ denim using similar selvedge denim. I'm not commenting on whether there is a quality difference here.
-When you do look at sources (including the previous one i listed above) the move from shuttle loom denim (selvedge) to projectile loom (non-selvedge) was generally due to cost. "To reduce costs, denim companies began using denim created on projectile looms. Projectile looms can create wider swaths of fabric and much more fabric overall at a much cheaper price than shuttle looms." So one could infer (which generally stands true) that selvedge denim/jeans cost more due to increase cost of the fabric. So you would think that a Coast in selvedge would cost more than a coast in non-selvedge denim. Again not commenting on the quality of the denim.
-With regards 3 x1 vs 2 x 1, I agree with you, i'm sure the feel and the look of the denim are major factors in what type of denim is chosen for a specific pair. I also agree that we can't really say which is more "valued" or "prized", but i think we can make comments on durability and weight. 3 x1 results in heavier weight denim (generally 10.5 ounces or greater) and 2 x 1 denim results in lighter weight/summer weight denim (generally 10.5 ounces or less). This site has great pictures of the weave and descriptions http://www.heddels.com/2014/08/know-twills-3x1-2x1-plain-weave/ . As that link states (and numerous others), 3 x 1 is a GENERALLY a sturdier more durable weave and therefore generally used more frequently.
To sum it up:
1. Shuttle loom denim/selvedge denim costs more to make therefore you can generally expect the denim to cost more
2. The selvedge ID, not only looks nicer, but serves a purpose to prevent the jeans from fraying at that seam, so maybe? more durable
3. 3 x1 denim results in heavier weight denim, which is generally sturdier
anything you would add?