Several years ago I purchased an Exped down filled inflatable sleeping pad. It is a more comfortable pad than Thermarest pads I've used in the past, so I would still recommend it. That being said, when I purchased the pad I remember Exped making a big deal about how their pads are filled with down as if this was a major improvement in terms of warmth (and possibly comfort) over other sleeping pads. At the time I bought this marketing pitch hook, line, and sinker.
However I was laying in my tent during a recent camping trip and I started to think about what possible benefit down really offers inside of an inflatable sleeping pad. As far as I know down is warm because it does a good job of trapping air and it is light weight. How would trapping air be helpful in an inflatable sleeping pad? The air is already trapped or the pad would deflate. Down's other claim to fame is being light weight. I don't know how that applies to a sleeping pad since not filling it with down would be lighter than filling it with down. Also getting down wet is a known problem and so Exped recommends that I fill my sleeping pad using the pad's stuff sack as an air pump so the moisture from my breath doesn't foul the down inside the pad. That leads to a bulky, specialized stuff sack that I didn't need for other sleeping pads. Another mild annoyance is there are two valves on the\h pad so that down doesn't collect on one side of the pad. This makes deflating quicker but it doubles the chance a valve will fail thus ruining my pad. The more I thought about it the more convinced I became that the increased warmth and comfort of my Exped pad was due to its substantially increased thickness vs Thermarest pads.
Does filling a sleeping pad offer any real benefit or is it just a marketing gimmick that exploits foolish people like myself who blindly accept down makes everything better?