It really all depends on the odds of rain on your trip, or how many rivers you will be crossing. i.e. what are the odds of your bag getting wet. I have a MB Down Inner Parka and it comes with me on every trip. It's perfect for 3-season backpacking. I also have an Outdoor research synthetic parka for winter/wet backpacking.
The odds of my sleeping bag getting wet are small. I put it in a BPL liner for any trip with no predicted rain. Sierra storms are intense but brief so that would be fine. If rain was predicted, or if I was fording rivers more than knee deep, I'd get a dry sack. The down jacket goes in an Event stuff sack with my NeoAir and baselayers, so it's not getting wet, period.
So I personally don't worry too much about a 'safety' synthetic layer. But I also only do 1-3 night trips as of now, so I'm never so far that bailing out in the unlikely case of wet down would be life threatening. Outside of pretty extreme conditions, you could probably survive one night doing situps/jumping jacks without getting hypothermic and bail out in the morning.
For a longer trip, getting farther out, I'd likely bring the OR parka as a safety measure.