In the popular mechanics thread it was claimed that at least one of these jump starter has a 30 volt battery. If this is true you could kill some electronics pretty quick. With the car battery in place, it acts as a HUGE buffer that will suck juice out of the jump starter (whose voltage will drop well below 30v under that massive load). Without the car battery you've just hooked your car electronics to 30 volts, which may or may not be fatal.

The 30 volt claim puzzles me, though. Don't believe it without further checking. A 12 volt battery would make much more sense (to me, at least), in which case it would be safe.