I had just one for well over 40 years until I took mine in for an annual cleaning one year. I was told even though it was old it was a good machine and that he could keep it running as long as one part didn't go on it. Since I didn't want to be without a machine I started looking at other machines. I didn't want a new one and happened to see a 15-91 that was for sale. I sat down and tried it out and fell in love with it and the vintage straight stitch machines, so I bought it. I have since picked up a couple treadles mainly because the price was right, I like sewing and antiques so they are right at home with an old oak ice box and an old oak crank telephone as well as several other antique pieces I have here.
Nothing wrong with having one machine if it serves your purpose.