I don't use a bag liner. I wear my Capilene and that keeps the bag clean, or at least cleaner than if I wasn't wearing it. I do have an overbag, an MEC Emperor Penguin and that does add warmth since it has its own insulation. I also have a light bivy sack. The three together is a bit of a hassle, so I now have a warmer bag.
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Don't get me started, you know how I get.