The VE's can and do fail. A friend of mine lost a rack of retics when his VE-300 failed wide open.
A thermostat is like any other electronic device; it can break. Running two in line is one way to help ensure that you don't end up with overheated critters. A cheaper option is to put a dimmer switch between the thermostat and the heating appliance. Set the dimmer so the appliance gets uncomfortably warm but not deathly hot if the thermostat fails. The critter's behavior will let you know there's a problem.
Note that dimmers do have power limits so while you can put one on a whole rack running heat tape, if you have a four-high stack of 8' enclosures with 120W RHP's, each RHP will need its own dimmer.
Reply With Quote