I used to not be into vests, but t b h that was before I actually knew how to layer/had a good layering system set up.
They're nearly useless without a good windproofing shell. For active applications where you're not worried about or welcome wind, a sleeved fleece jacket works great. For static applications you don't want the venting.
But as a third/second midlayer in variable conditions? If you can take off the shell to put them on without heat loss being a concern (most urban applications) they're great. Need just a little extra heat sometimes, or out all day and feel cold after the temperatures dip? You can stow one in your bag and pull it out. And then the lack of durable fabrics, extra pockets, wind resistance that makes most vests pretty useless as an outer layer doesn't matter, saves weight and pack space.
Even while /out/, they can add a good bit of insulation to your core without compromising venting/breathability. There's a reason a lot of jackets are like 80/60 or 60/40 insulation in the core/sleeves.