this makes a lot of sense to use a liner for extra warmth and to keep your bag cleaner--especially if you sleep in just boxers, washing a liner once in a while is much easier than washing the whole bag. When I sleep, I like to have thin tops and bottom on, just a personal thing, but the added plus is that if I am slightly sweaty and or the usual funkiness that comes from wearing something for a while, even sleeping, I just wash them--an easy wash and dry. In boxers, the inside of your sleeping bag over time will get funky and the oils from your body will go on it, a liner will keep your bag clean plus have the added warmth bonus.