You can think of a spare as a backup controller or as some of us do it becomes the testing controller on the workbench and a backup if needed. With all the money I have invested in lights, props and active controllers, the cost of one extra controller is a pretty small investment and since it can used on the test bench it is also a useful investment. I also consider it insurance. I try not to have anything that would kill my show not have a backup. I even have a backup FM transmitter, albeit an older one that I used in the past. I started with LOR, migrated to Lynx AC, migrated to Lynx RGB, and now use mostly Falcons. I sold all my LOR which paid for my Lynx AC at the time, actually for more Lynx channels than LOR channels, sold most of my Lynx AC and bought Falcon pixelnet stuff with those proceeds, have now been selling Lynx smart string stuff. When I can sell a used controller for 75-90% of what they cost me I don't consider that a bad deal, or a bad investment or a waste of money. Sometimes I even sell stuff low to get a newbie a head start.