I’m no expert, but a recent poster in these forums had a specific experience in the mountains, which beats theory any day, IMHO. I edited it slightly, so I am just paraphrasing:
I went to fleece exclusively after topping out on Shoestring in -10º F (before wind chill) temps, with 30-40 mph gusts. We were working hard and sweating heavily while moving, and my Capilene 3 and R2 fleece let it out. My partner was wearing a Micropuff inside his shell, and it was a frozen mess, stuck to his shell and not warm at all any more. It breathes, but not nearly as well as the R2 fleece.
[NOTE: “R2 fleece” claims to be the most breathable fleece on the market, similar to the R1 hoody material. I have no connection to Patagucci, but they say “We've worked with Polartec® to further refine this exclusive Thermal Pro® polyester fabric, now significantly lighter, more compressible and breathable, a touch warmer, and with better stretch and a softer feel.”]
[OP continues:] When climbing you go super-hard for a while, then sit around belaying, or messing with gear, and then go again. These cycles of activity wreak havoc on any clothing system, and when you are covered with slings, ropes, and a harness you can't add/shed layers easily. So, we need maximum flexibility. Fleece breathes really well, so when used under a shell you can zip the shell up and down (as far as the harness allows), and the air movement cools you down fast. Then zip it up and get warm again. The R1 Hoody is even more alpine specific - the cuffs stay in place under ice axe leashes and big vertical reaches, and the flat, non-zip lower Cap 4 section fits flat and tight under a harness. My NanoPuff Hoody weighs the same as my R1 Hoody, offers much more warmth, and blocks wind and some rain. If you can take it on and off, it's great. But, if I put it under my shell like my R1 Hoody - I'd overheat while moving hard, as it doesn't vent or breathe very well.