I think Rich from Reptile Basics covers this topic pretty well in his FAQ:

"Safety backups - When and why they are needed

No matter how much you spend or what model/brand you get your thermostat CAN FAIL! A number of things can lead to this happening: power surges, children, overloading, rat chewing on a poorly placed probe, bad luck or karma, etc. BE READY, you can use a simple, less expensive thermostat as a back up. Odds of both failing at the same time are much, much less! In many cases this may not be needed. If careful consideration is given to the maximum heat possible to a given cage or tub you may find it impossible to get to fatal temps. In smaller tubs, like 5-12 quart sizes, you may find that even with low wattage heat tape that your tubs could potentially reach fatal temps easily! With the VE-100 as you backup and either the VE-200 or VE-300 as your primary the units will stack for a very neat, organized appearance.

Most failures we hear about are actually not failures of the thermostat, but failures of the probe. Often a probe can be dislodged with constant moving of tubs in racks or in cages. Some thermostats can not differentiate between normal circumstances and when your heat element is not working properly. The Vivarium Electronics VE-300 model comes with a full set of alarms to de activate the heat ouput and sound a loud buzzer when there is an issue. For example- with most thermostats if the probe is dislodged from the heater the thermostat will simply turn up (or on) the power, never realizing the heat element has surpassed the set point. With the VE-300 if the setpoint is not reached for more than 30 continuous minutes the alarm will let you know- and shut off the power output!"