Well, yes,*@LaMarschallin, but have you ever had a sheet that has literally worn into holes in any part?
No, not holes but I have had some go very threadbare. Admittedly, they were ancient ones that grandparents passed on when I first set up home ( I'm in regal company - Joanna Lumley says that you sleep on 2nd hand sheets if you stay with the queen. Mind you hers were 2nd hand from Britannia when it was decommissioned. Mine, er... weren't.).
Was in a position to get rid of them then though.
I have seen sides-to-middled sheets used as undersheets.
But you're right. You wouldn't want your sides-to-middle done by actual middles!
(Remember in Little Woman the sisters making hemming sheets more palatable by naming them after continents and talking about them as they sewed?)