I assume you're thinking about the PHD Delta sleeping bags?
They look like pretty specialised bits of kit to me. The lining should limit how damp you get the down in you get into your sleepingbag whilst still soggy. So unless you're going somewhere that's very cold with properly unpleasant conditions, and unable to find the space to actually change into dry clothing because you're sleeping on a 6" wide ledge several thousand feet up, you don't need one.
You're probably best off asking PHD who they make em for.
I can't stop messing about with the PHD DYO program and came across the Drishell inner option which intrigues me. I was thinking of going for Drishell inner and outer for use in a wild camp bivvy.
Are you going to get into your bag whilst wearing wet clothing or covered in snow?
If so, don't.
This nicely removes the need for a water resistant inner
You'd be better off with a synthetic bag if you're going to mistreat it like that. I can see the use of water resistant panels around the opening (for breath condensation in very cold temperatures) and maybe inside the foot end but beyond that its a bit of a weird bit of kit for very specific environments.
If you were bivvying, you might perhaps be better off with a proper shell outer to keep the wet off (because drishell won't hold up to rain) and plain microfibre in and out of the bag to keep it light, cheap and slightly more breathable if you use it without the shell bag.
Your comments align with my thoughts on the matter. I bought a RAB Alpine 600 bag recently, mainly because of the Endurance outer. Unfortunately, it was a little too short for me, so I'm having to re-consider my options. It's driving me round the loop I want to travel as lightly as possible, but on the other hand I don't wish to destroy an expensive down bag.
I'm generally in favour of water resistant outers, but they're not actually critical. There was a thread on OM in the recent past in which a number of posters said they preferred not having a water resistant outer in the interests of increased breathability. All of the down bags I have lying around here (3, of which 2 are mine) have standard microfibre outers and they've been fine. My down jacket does have a drishell outer, but that is far more likely to be exposed to the elements than my sleeping bag.
As for destruction... getting a down bag soaked won't do it much good in the short term, but you can dry it out and fluff it up again when you get home. Might cut a trip short, though.
Waterproof bags are heavier than non-waterproof bags, all being equal. So if you want to travel as light as possible then avoid the waterproofing and on most trips you'll be ahead. If you take a bivvi one time in a while that'll be heavier, but mostly you're ahead.
Also remember that a bivvi bag not only keeps your bag dry, but keeps it clean. Last time we used our BBags was in a bothy with a sound roof but a mud floor...
It's ~ £30 to get a down bag cleaned professionally, and if you susbscribe to time == money then much more than minimum wage and you're ahead compared to doing it yourself... Clean bags work better than dirty ones.
I'd agree with pretty much all Serriadh has said: Drishell in and out is an option rather than default because it's not something that makes much sense to most people, most of the time. That doesn't mean you shouldn't have it but mostly I think you'll add cost for little benefit.
I want to travel as lightly as possible, but on the other hand I don't wish to destroy an expensive down bag.
