Clothing & insulation for every active sport all follow the same principle: protect the core (ie torso).
For example, surfing wetsuits have used variable combinations of neoprene thickness for decades. Typically, winter wetsuits will have either a 4/3mm chest/torso, while the arms & legs are 3/2mm. More obvious are 'spring suits'; not only do they use a 2/1 ratio, but they have both short sleeves/legs. (Or, as my non-surfing wife refers to them, 'jumpers'.)
As Jerry notes, the name of the game is to insulate your torso. Having insulated arms (as in a jacket) makes as much sense as having insulated pants. IOW, if you have a down jacket, why not down pants? The net effect is the same.
Vests provide a practically perfect layering component. First, you have your long sleeve/leggings base; then a SS shirt & short pants. Next is a lightweight fleece/wool, then perhaps a windshirt and or rain shell.
Last, but not least, is the down vest. If used in conjunction with the fleece/wool (wear the vest underneath), along with the wind/rain shell on top, you've got a great system that can take you down to freezing using Richard N's clo/activity charts.