Sorry, rigarashi, I must correct you. the thermistors used here are Negative Temperature Coefficient. High resistance = low temperature; Low resistance = high temperature. Open circuit means low-temperature fault; short circuit mean high-temperature fault.
That said, you are correct that a failure of the firmware or the hardware on the Printrboard can lead to an uncontrolled amount of heat going to the heater(s). And if you really want to be safe, yes you should use other means to detect and deal with that problem. Maybe smoke detector, maybe bimetallic temperature switch, etc. All of the "old timers" on this forum strongly recommend never leaving the printer running for a long time while totally unattended (like if you leave the premises for hours at a time or go to sleep). Personally, I never leave the printer running for more than 30 minutes or so without actually going to look at it, and I'm usually inside the house for just about the whole time. Lots of people have cameras set up so they can check on their smartphone.
BTW, the firmware is called "Marlin" not "Merlin"
Printrbot Plus operational January 2013
Brass threaded rods (5/16" X 18) & nuts for Z axis
GT2 belts & pulleys
Cable chain to reduce probability of fatigue failure in wires
E3D V5 Hot End, 0.4mm nozzle, also 0.8 and 0.25 in use occasionally
PB fan mount + 40mm fan -- using printed mount adapter, not the E3D supplied fan
Injection molded extruder gears
Optical Z "endstop" (custom designed and built)
Have used many pounds of T-Glase filament. Now also doing some work with Ninjaflex SemiFlex
Print on glass with Scotch Craft Stick or other glue stick
"My next printer is..." Prusa i3 MK3, upgraded to MK3S