At the risk of fanning a fire, I actually quite like the jacket and jeans look (though not all of what you describe).
Yes, it does cause cognitive dissonance, though generally good examples would use a less-formal jacket (what someone might call a sports jacket, but generally a lot more fashion-forward) to reduce this somewhat. But this is part of the idea. Going out is a time when looks matter, when you want to wear something that isn't too normal. Traditionally this is the function of black or white tie, but that's obviously inappropriate for a bar or informal party in a modern world. A suit would also be far to formal and look out of place. They key is not to look formal, but to show some effort. Dressing down a jacket is one way to do that.
Open-neck shirts I'm also a huge fan of. It's not really about being conformist or non-conformist, it's about looking laid back and relaxed. It's more comfortable (at least I find it so) and less formal. A tie, even more than a suit, would look out of place at an informal party or going out to bar. Also, while it reduces the variation you can put into an outfit, it means you can customise elsewhere. You see jackets with embriodery, more-patterned shirts and a whole host of jeans customisations. To add a tie would make it fussy and unbalanced, and by taking it away you can add more detail elsewhere.
I agree that untucked shirts look silly, unless they're one (of the few) that's cut flat across the bottom and short, specifically so that it can be worn outside of trousers.
I think wearing a normal suit jacket with jeans doesn't work as well. For the really outgoing, I've seen tuxes worn with jeans that look great, though usually somewhat customised (some designers do great examples designed to be worn like this - Holland Esquire springs to mind). Though more normally, you'd simply use a less-formal jacket (for example I have a linen one-button jacket to go with jeans). I even quite like it with a jumper or t-shirt, rather than a shirt underneath. Basically using a jacket as I would any other light summer coat.
I appreciate many people will not like this, which is fine. Were I going to a fine restaurant, the theatre or to work, I would dress more smartly. However I would ask, what would you wear when being dressed smartly - even wearing a shirt, tie, slacks and a sports jacket - would look out of place and be inappropriate? Or would you just never be in that situation? To me, dressing appropriately, while still showing your own style and flattering your shape, is the key to dressing well. Personally, I feel this applies as much when dressing down as dressing up.