Hmmm. To my mind there are substantial advantages to a sleeping bag liner. The first, que no? is that the general foulness is contained by the liner, and it's a lot easier to clean the liner than deal with a down bag. You can wash it out every day if you're in the humour, and it'll dry in minutes.
Secondly, yes in my experience a silk liner gives you an extra three to four degrees of protection. Silk is important here - it's a terrific natural fibre, with the ability to both cool you in heat and protect you against cold. How those spiders do it is entirely beyond me.
A good quality silk liner will cost you around £40 - but this is money well spent. It will repay you in both camp hygeine and warmth. Cotton liners are a false economy - they'll pick up condensation, are heavier than silk and the money that you'll save is far outweighed by their poor performance.
I've a couple of PHD bags - one thirteen years old, and the other a tad younger. They've been with me in conditions far beyond The Challenge - I've always used silk liners to complement them, and in my own opinion a good liner protects your bag, gives you sleep comfort - absolutely critical on any extended trip, as sleep deprivation is to my mind the single most debilitating of conditions - and increases your sleep system of choice's performance.