The reasons I'm collecting them is that the cases are fairly uniform in size and shape. Therefore, they are easy to shelve and store until time to set up a class. I load a trailer with bins of fabric, patterns, mats, cutters, scissors, thread, etc., and and assorted size and weight machines. I take my show on the road to teach 4-H kids how to sew.
When you need to lift 20 - 30 sewing machines, some of those units get pretty heavy. The kids unload and reload the trailer at the class site - but I load at the start, and unload when I get back home. I'm not young, or healthy, so it's a chore for me. Even the healthy kids have trouble with the heavy machines.
Having all the same style machine will also make it easier to teach a large class. The Featherweights are durable, and pretty much kid proof. I would like to have a total of 30 of them. (I have a long way to go.) As I get a featherweight, I can donate one of the older flea-market odd balls to one of the kids who really has promise in sewing.