Why? Used motor oil is, well, used. It contains metal particles from the engine and combustion byproducts, which is why it was replaced in the first place. Granted, most lubrication applications aren't the marvels of precision parts moving at high speed that a modern engine is so can probably make do with poorer oil, but still.
You can buy industrial lubricants in bulk for pretty cheap so that unless you use huge quantities of it, it shouldn't make much difference.
As an aside, my aunt's husband worked more or less his entire career in a heavy truck repair shop. And he had an oil burner heating his house (you can see where this is going, eh?). So he got used engine oil for free, the shop was happy to get rid of it as disposing of it properly cost money. I think burning used engine oil was illegal already back then due to the pollution, and nowadays I think they have some government mandated accounting system to ensure that the same amount of oil is sent to proper recycling as comes in.