While I would allow my kids or sister to use my machine, with me right there, I sure wouldn't let someone "rent" my machine. I find that even when I change type of threads from the 2 I routinely use, the machine needs adjusting. Then there are all the issues already mentioned.
There are a couple of quilt shops in my area that have machines for rent--after a certification class, etc. But having talked with one owner, their business model includes expenses for repairs, liability insurance, utilities, wear & tear on the machines--none of which I've built into my expenses as a long arm quilter--so figure it' cheaper for me to quilt for someone than t rent my machine!