You need to have a machine to yourself for when you want it. They can get a cheap machine to do what they need or even get a thrift store find. If yours is yours and you leave a project on it so you can go do something else it's nice to know it'll still be there when you return.
You'll probably spend alot of money on the one you want and getting the tension just the way you want and knowing that the last time you used it the needle was good are things that will be valuable to you.
You don't want to have to adjust it or change the needle each time because you don't know what or if they used it. I'm sure a horse blanket would dull a needle.
Last year when I started quilting again and my daughter wanted to do it; I gave her my old machine; I bought a new one for sewing, and one Viking mega quilter for quilting; I like multi-tasking and this way I dont't have to keep changing out the thread in the middle of a project. My daughter doesn't even sew in my room; it's small and it's mine.