- superior ventilation ... big holes to put it simple ... think about it whats one of the places most likely to sweat thats not very breathable? ... the sleeves especially near the wrists ...
- weight efficient ... as a ratio you requires less fabric to cover more of the body with a vest, all other things being equal
- warms the core ... and it does it while allowing good ventilation
- doesnt soak up as easily ... when yr always taking on/off your layer in the rain/snow, whats the first thing that always gets damp and soaked ... the sleeves and the hem ... we cant do anything about the hem, but a vest doesnt need to worry about soaked sleeves ...
- dries quicker ... same as the above ... when youre using cold/wet weather dying techniques (hawt nalgenes, wear active till "less damp", etc ...) vests tend to dry faster because of less surface area and greater heat around the core (natural or artificial) ...
- cheap ... theres a lands end down vest for 12$ thread on gear deals right now ... want a fleece vest, buy a cheap one from lands end ... or even better take an old well used fleece and cut it up when the sleeves wear out ... just as people make shorts out of worn pants, make your own fleece vests
- less warm in the arms ... for static use its an issue especially if its cold, windy and wet
- fleece .. as an active layer when its cold, or wet, or when your traveling slowly due to the terrain ... also for climbing or other activities when having the arms unencumbered might matter ...
- synth ... as a semi-active layer same as the above ... or a quick static insulation layer for quick stops ... remember because of the lack of sleeves it will get less damp than a jacket when you take it in and out around snow/rain all the time ... for long stops you probably want a full jacket
- wind vest ... for very high output activities such as running, biking, brisk uphill ascents, etc ... gives core wind protection but leaves the arms/armpits very breathable
- down ... least useful, best used as a booster layer for quick stops or a booster puffy layer