I do very little backpacking – I hike to get to climbs.
It's that sport where these pieces shine. 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.
When UL backbacking, just bring down or synthetic insulation. For instance, 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.
I went to fleece exclusively after topping out on Shoestring in -10º (before windchill) temps, with 30-40mph gusts. We were working hard and sweating heavily while moving, and my Cap 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 fleece.