They are a problem. I know if I'm faced with two equal machines, one in a cabinet and one in a case, I'll take the one in the case first just because of the space issues. Donate them, Craigslist, Freecycle, anything like that. Beats burning them anyway. I don't get too concerned about the particle board ones but the earlier ones made with quality materials are worth preserving. Chalk painted end table beats ashes. I have far less machines but am already faced with the same dilemma. I've passed on several treadle machines at giveaway prices just because of the space issue.
I'm building cases for my homeless machines and Cari has given me a nice table someone else already modified. It will get a tray to hold my new case bottoms. I'll have the best of both worlds then. A table mounted machine without the storage issues plus I can rotate machines in and out of it at will.
Rodney