In reply to Dangerous Dave:
Must be one of the most often asked questions.......?
If it's warm enough for belaying for more than 1hr, it's too bulky, needs to be carried in a pack! Or you have one between 2, second wearing it, pass over at belay. Typically more expensive to trash also!
This system means all heat generated by leading is often lost, before a big puffy thing can be worn, then having to reheat oneself.
Something is better than nothing. A compact, light thing hanging off harness, can easily be carried by both, donned immediatly before evaporated heat loss takes place! If a monster pitch has to be lead, both can be worn/carried by second.
Winter climbing is inherently a cold enviroment, accepting this, what gives you the most fun climbing?
Stuart