I'm not an expert in contactors, but there is a reason why the typical aircraft "master" contactor and the starter contactor are different, even though the starter current goes through both.  The master contactor isn't expected to open the starter circuit, while the starter contactor is.  Therefore, the starter contactor is built with a higher-force return spring to guarantee the contacts won't weld when the starter is shut off.  To energize it they use a coil with fewer turns of larger wire.  Hence the coil draws much more current, dissipating more heat than the master contactor.  Works fine for short duty cycles, but will overheat if turned on continuously.  The master contactor, wound with more turns of finer wire and using a lighter spring, will turn on and off more "gently," but won't overheat.  So, if the starter contactor (or starter switch) does stick on, will the master be able to break the circuit?  Probably.

 

The starter contactor, though, is designed NOT to be the "weak link" in the starter circuit.  If it does stick closed, the current draw is not "huge," but is probably 100 amps or so, and that won't start anything on fire.  The starter will definitely burn up before the contactor will fail.  It's call "intermittent" because it will eventually overheat because of the coil current draw, not because the contacts are weak.  Incidently, way back when, car electrical systems were designed so that the battery would go dead before the starter burned up - I don't know whether or not that's still true.  Since an airplane battery is maybe a quarter the size of a car battery, I would guess it would go dead in time to save the starter, but I'm not sure.  The worst case is if the engine starts, the contactor sticks on and the pilot doesn't know the starter is still engaged.  In that case the alternator is charging the battery and then I suppose the starter will burn up first.  What to do if you know the starter is still running?  I suppose you shut off the master and then turn off the engine.  Right?

Gary