This subject could easily go down the road of "hand stitching vs machine stitching" but I hope that is not the case. First we have to make some assumptions, one of which is that the best grade of materials is used in each case.
It is generally accepted fact that many materials including leather when laminated are stronger than a single ply of the same thickness. Therefore if strength and durability is a desired factor lining could or would be considered a benefit.
If the lining has a top grain and that grain is exposed on the interior, then the abrasive wear on the item contained is greatly reduced, hence another benefit.
A nicely lined item has the edge when it comes to pure aesthetics so that is another benefit, perhaps small, but none the less a benefit.
It has been a long standing practice with my work that I line everything I make. It has become somewhat of a trademark and as such is definitely a benefit for me as an individual.
The foregoing expresses my personal opinion and should in no way be taken as disparaging on unlined work, because as shown on this board there are examples of beautifully done and certainly strong and durable unlined goods made by members who display here.
Paul
Great post Paul and exactly the type of observation and discussion I had in mind when I started this thread .
Your experience making sheaths sets you apart from most here , so your views hold a lot of weight .
Even though I don't make fixed blade sheaths and what I do put together are at the other extreme of design from yours , I've really listened to and taken on the advice you give out here .
For example double siding my leather by gluing to thinner pieces together like you suggested in a reply to one of my questions once really changed my sheath making
Your post above covers most of the things I was going to mention , so you have taken the wind out of my sails a bit

I have over the last couple of years really moved into chasing performance over aesthetics ( please don't get me wrong , looks are very important , but never at the cost of performance ) and for those reasons double siding or lining a sheath does have performance benefits exactly as you mention .
Lately in my never ending quest to simplify I've moved over to just about exclusively making Roughout sheaths . Performance wise they meet my requirements in the most minimalistic way and one of the biggest byproducts of Roughout is you get an instant lined sheath which no one ever really mentions .
I'm interested to here you think their is less abrasive wear on the knife when it's in contact with the smooth side of the leather . My brain says that this must be the chase , but I have no evidence really to back up that idea and I suppose that was some of the reason for this thread to see if others think that a lined sheath protects and performs better .
Ken