Thanks, BillyBob58. That was an eye-opener.
To be honest, there are three reasons that I use synthetic insulation over down:
1.) I'm not a very experienced backcountry camper. I feel that I have a good handle on the basics, but I'd much rather have something that will work despite my potential screw-ups. I know how to build a survival fire, but I've never had a chance to attempt it in 33* sleet, so I'm not completely sure that I can under circumstances like that. Synthetics give me a bit of a safety net under those circumstances.
2.) Cost. Plainly put, synthetics are cheaper than down items, on the whole.
3.) Age and general fitness. I'm under thirty and in decent shape; a pound's difference in the pack isn't going to kill me over the distances I like to hike.
All of that being said, down is a much more efficient insulation under most backpacking circumstances. It just isn't something I'm going to be using in the near future.