It's not always a case of "pawning off". A friend of mine has an ancient 30yr old, toothless pony who gets used as a companion. She's calm, quiet, easy-going and makes a great buddy for a recovering horse or a high-strung one. Doesn't crib, chew, destroy fences or attempt to escape, even if the gate is wide open.
She's not exactly low-maintenance as being toothless all her meals have to be soaked and she cannot eat hay in any form other than soaked pellets. Her companion skills and non-destructive ways out-value her feed needs though, so people "borrow" her when they need a companion.
She's in-between "jobs" right now so she hangs out at the barn and keeps my horse company (he shares his alfalfa with her, she loves the taste, not that she can actually "eat" it). If I was to bring my horse home instead of boarding him, I'd bring her too. I won't keep a horse alone and I don't have time to ride multiple or deal with a horse that is destructive or needs training-refreshers in addition to my own horse. ANY horse I got to keep mine company would cost me $$$, why not take one that won't cause me the slightest headache.