I see gloves (mittens when it's really cold) as part of the big picture. How warm a glove I wear is related to how warmly I dress overall relative to the starting and expected temperatures I am going to ride. How much climbing also factors in as some of my warmer gloves do far better than others for hard climbing. My hands run cold so I have to factor that in.

I love full fingered unlined MTB gloves for serious climbing and wear them over a wide range of temps, from 80F to mid 40s. For me. the toughest judgement calls come in the low 40s to low 30s range when sweated up gloves can be very cold; Chopper mitts and liner mittens often are my choice because of the versatility. (Inner mittens out up to 70F+ works really well and in down to the low 20s. I used that combo below 0F when I was a young stud for a 12 mile commute.)

Aside - there is a real plus to riding bikes that operate well with mittens in really cold weather. Yet another reason fix gears (with good brakes) rule in winter.

Ben