Here's my problem with using them...
Welding cable is generally smaller than other cable, for a given ampacity, which has many advantages. However, the temp rating is based on the entire circuit including terminals and devices. At a given ampacity, the welding cable will be warmer, the insulation may be rated for higher temps, but the teminal and device ratings will still prevent you from being allowed to exceed a certain temperature. At this point you're back at square one using the same size cable.
Also, what voltage is welding cable rated for? Most welders are well below 120V to ground, and the insulation may not have the dielectric strength for 600V at higher temps.