I was at Sage when they decided to have the installer not auto check the demo companies. any longer (late 3.x). I asked Why because I think demo companies should be auto checked as well.
The primary answer was a call came into Support where an end user self upgraded their 3.x to a later 3.x in-place (parallel migration was not an official concept then) and their live company code was ABC which did get overwritten with demo ABC. The other reason was demo data will install demo Sales Tax Codes and Schedules (you know the CA, CA BA, WI,WI MIL) and people called to say ""we didn't set that up why is it there"".