Holiday, I am sure you likely know this, but these days I NEVER back anything. I believe in letting the buyers choose who they want to do the restoration, how much restoration they want, etc.
The ones I auction backed were backed by the consignor, or someone further back, but not by me.
As to why someone backs an inexpensive one-sheet, it is so it looks its best on the wall. They are not thinking "investment" and these almost always sell for less than the cost of linenbacking alone.
Bruce