who uses one?

Im thinking about getting my 6 slot rack in the next couple months and im trying to decide what i want vs what i need. I know we need to get a *good* thermostat and im thinking about getting the ranco since it shuts off when temps get too high but ive also read people having a thermostat (such as a johnson (?)
and then having a back up ranco attached to it..

how nessesary is that and who has it currently?