The PMDX-126 e-stop connector cuts the step and direction signals when the e-stop is triggered. My understanding is that this will probably stop the steppers faster than if you just cut the power and it doesn't rely on software (well, it probably relies on the firmware in the BOB but that's a lot better than going back to Mach / LinuxCNC to handle e-stop). My concern with having the limits cutting the power is that it will happen comparatively frequently compared to real emergencies and I've read in a few places it's not a good idea to just cut power to steppers that are in motion due to back emf. I wouldn't take just my word for it though. I assume if your bob can't cut the step and direction signals it would easy enough to stick in a relay to perform the same function.