I don't see what the big mystery is here. 
  
I just bought a Montbell U.L. inner jacket.  Why?  At only 7 oz, it's lighter and warmer than even my lightest softshell, which it replaces during stable, warmer weather.  It's awful nice to be able to go out with basically a couple of silk weight Ts, biking arms, and the Montbell during a nice summer high pressure.  Yes, there is an 'overhead' with the fabric, but Montbell uses 800 fill down; pretty much the best you can get.  It also makes a nice wind layer during the morning/eve for the conditions just mentioned.  It's also a great belay jacket;  so compact you can hang it on your harness.  The FF garments are just a bit too bulky for that.  During slightly cooler or more changeable weather, I can throw the Montbell in with my softshell, which I'm more likely to use while moving.  If it gets colder still, I go with my FF down jacket, which is much warmer but weighs about a pound.  Another advantage to the Montbell: if we get a REAL COLD snap, it can supplement my FF down jacket.      
  
As for why people wear these down sweaters around town, I can only guess that they, particularly women, prefer not to look like the Michelin man (as with puffier garments).  In addition, these minimalist down garments are so light you can't even feel you've got it on.  I suspect people find that warmth without bulk feeling appealing.     
  
The Montbell I just purchased fits like a grocery bag.  Slightly too baggy in the waist and too tight in the chest.  It's for utility, not fashion IMO, so I don't care that much (plus it was damn cheap on sale).  It's also delicate; there are other, less pricey jackets I can beat on around town.  Perhaps the Patagucci offering is stylish enough to be appealing for urban use. But frankly, who the hell can afford Patagucci?