Yes, the liner (well, I’ve only used the HW liners, so can’t speak to others) + blanket over top is lighter than two blankets. It’s also less bulky, which is a bigger help for me, easier to fold up/carry/store. I have a very nice heavyweight blanket, but it takes up as much space as 2 midweights & is a full armload.
I also like that with liner + blanket I only have one set of straps on the horse & I never have to adjust them. When I was layering multiple blankets, I was having to let out or take in belly straps to allow for extra bulk.
I have not had liner get wet (despite hurricanes & winter storms, & my horses don’t have stalls, just their shelter). They are cut shorter so they don’t extend pass the blanket. Even if an edge gets wet, the edges have plenty of airflow next to an animal producing tons of heat so they dry themselves quickly.
With your plethora of blankets, you could easily designate a liner combo for super cold weather that just doesn’t get taken apart, as you mentioned above.
For me, the biggest advantage was the liner just gave me more flexibility, as it can be used with just about any blanket, for way less $$$ bc I just can’t & won’t spend $200-400 on a horse blanket. Saving me time & effort was a completely unexpected bonus. But ultimately, for anyone boarding, it will also depend on your barn staff too.
Good luck & hope you find your perfect solution! -another winter hater