Many gear lists include them. You are going to see people recommending them all the time (often with a link where they get an affiliate marketing kickback if you buy one).
I was in love with liners (or at least the idea) when I first started researching and buying gear. So often someone writing about hiking would wax poetic about how they loved their liner (with a link to where you could buy their favorite).
I never quite bought one (they are expensive!) but kept planning to buy or make one. I kept thinking about them.
Then I had to sleep in a tent with only an airplane fleece blanket for warmth and I was surprised at how well it worked. Only a pound or so and surprisingly warm. I was sold on the idea.
Then I hiked a section of the AT with a car quilt (the Costco down blankets) m, leaving the sleeping bags in the car because the weather was warm. I was getting more sold. A liner and a light quilt and I could be warm —to 40 or 50 degrees.
Every time I started researching or trying to layer gear, I started getting reviews where the liners were either:
a) light and not warm compared (to a base layer), (review after review of people complaining how little warmth they got from the liner) or
b) the liners/additional layers were for the most part too heavy compared to a base layer and better sleeping bag. Liners ~ warmth ~ weight, but ounce for ounce down is a lot warmer.
Warmth always requires weight. Nothing gives you free warmth without weight. And multiple layers weigh more. There are some great multiple layer sleeping bags —they just weigh seven pounds.
You can make a great liner out of a two pound (32 ounce) fleece blanket, folded over and sewn down the side. It will work well with a quilt or sleeping bag.
Sounds great until you realize that a feathered friends Egret will keep you warm to 20 degrees and weighs less than 30 ounces.
The comparison is a sixteen ounce quilt + two pound liner or a bag that weighs less than just the liner?
Not that there isn’t a place for warm weather gear
I’ve got a cheap light bag (weighs about one pound, 850 fill power) for warm weather.
I also have a real bag (comfortable to 20 degrees, to 17 zipped together with my wife). In addition I have a base layer to add a little warmth and keep my bag clean/protected from my skin oil and for daytime wear when it is cold.
[Definition. A base layer is hiker speak for a silk or wool set of long johns and long sleeve t-shirt used like pajamas at night or for extra warmth while hiking. They are part of the essentials through hikers use].
So I gave away the fleece blankets that I was going to make into a liner, both of them (yes. I had too many blankets).
Bottom line: Two pounds for a liner? Just not worth it.
As for 12-16 ounces for a liner? What do I think?
If it is really cold out I can layer my down bags together. The cheap bag is warmer and and weighs the same or less
EE even has a chart that I’ve added to my suggested links for calculating layered warmth.
A true 0 degree bag might be warmer but I don’t camp in that sort of weather and I just don’t have a reason to spend the money for another bag.
That isn’t to say there isn’t a time and a place for liners.
For car camping and van life they are perfect sheet replacements. Many are just sheets folded over and sewn down one side. In this use the weight doesn’t matter. Your car won’t complain about twenty extra ounces.
For hostels (especially European ones on the Camino), I’m all for liners/carry your own sheets (and insect shield treatment for them too).
That lets you sleep in hostels (many European hostels require you to bring your own sheets) and insect shield treatment protects you from fleas and ticks and bed bugs.
Outside of that use, basically other than a liner as a sheet that does not tangle, liners are kind of, err, mistake. If you want them as a sheet layer, more power to you. That is a perfectly valid position to take.
For for those who disagree and who are pushing them for warmth with links to where you can buy them, I can only say “You got this.” (The generic response to people who don’t really want to listen and who just want to push an idea/sell you something).
If you know what I mean.