I have a few 10 gallon tanks that I put up and take down according to what I need. They get used for hospital tanks, for quarantining new fish (when I know ahead of time that I'll be getting them), and for spawning/raising fry.
Since I have a 55 gallon and can clone a 10 gallon cycled tank with no problems, I do not keep an extra tank set up when I don't need it for a specific purpose.