In cold winter conditions and very moist conditions, using a synthetic over bag on top of your down bag is a really great way to keep your down dry. Climashield synthetic insulation deals with moisture very well. Down does not. Even modern treated down. In cold conditions a scenario is created that over time will wet out a down bag even if outside all the moisture is locked up as ice and snow. When you sleep in a sleep system your body is expelling moisture constantly. Quite a bit actually. If it is warm, the moisture just moves on through. When it is cold, the moisture will collect on the first cold surface it reaches. Unfortunately the first cold surface is going to be at some point inside the down fill of your sleep system. If you put a Climashield over bag over your down, that dew point gets pushed out of the down and into the Climashield where it can be easily dealt with. Additionally, in really wet conditions it is almost impossible to stay dry just from dew and blowing precipitation. A synthetic overbag will catch all that incoming moisture too. Climashield APEX will continue to keep you warm when it is wet and dries fast. I build my APEX over bags with  a snap to connect right onto my down bags so it all essentially becomes a single unit. My over bags also double as an insulated poncho just by opening the footbox and securing it around your shoulders. This again provides an opportunity to carry gear that has more than one purpose, either as a safety net or so you can go ultralight and maybe leave the jacket at home....or carry a lighter one.