I have to pitch in as I am also interested regarding people’s preference.
Winter vs Summer sleeping bags/liners:
Winter temperatures average below 10 Celsius with occasional lows < 0C. If you need extra padding at temp below this then I think any liner will be useless. In my experience, if synthetic material for insl was on par with down then I would recommend it. I have never employed any liners in any Antarctic or similar (snowbound) environments due to this fact. Rather try and choose a sleeping bag wisely (For an in depth review see: Choosing A Sleeping Bag – Feathered Friends ) Also remember if you pile on clothes the air within the sleeping bag takes longer to heat, if it heats at all (this is dependent on body heat). I take a reusable chemically activated pouch to heat the inside (https://heatinaclick.co.za/collections/orthopaedic-heat-pad) of my Deuter down sleeping bag (EN: Comfort 4C +/-6C) on subzero nights due to its -2C lower limit.
Summer Sleeping bag liners:
Now this is tricky…I have come to realise that a 4-10C comfort sleeping bag can get really uncomfortable in summer. I opted for a 500g lightweight Deuter sleeping bag and tested it out along the garden route…waaaaaay toooo cold. So in this case I am with you on the liner. The problem is do you dock out the R3000 for the expensive SeatoSummit top of the range and keep it lightweight (keep in mind this is 500g +350g=850g) or throw lightweight out the window and take along a fleese liner? Considering that most lightweight <500g SB are R1000, should you not just rather buy a summer sleeping bag For less than the R4000 within a range of 15C comfort and stick with it? I do not have the answer…sorry
To help you make up your mind I can only offer the following which helped me decide:
Review of best 2019 Sleeping bag liners & explanation of liner materials>
Review of Sleeping bags & an explanation on sleeping bag ISO/EN ranges: