Chu Chu:
Other than that, I'm not sure why arcing would be a bigger problem in unpressurized areas. But there's probably one I haven't thought of.
The effect you refer to is described by Paschen's Law. Here is a curve describing breakdown voltage per inch vs air pressure:
Breakdown Voltage | Paschen Curve | Altitude and Pressure | Corona | High Voltage Spacing | Insulation | from High Voltage Connection
Short of [no pun intended] an complete arc, this breakdown voltage also affects something called corona discharge around charged conductors. That's the crackling noise you hear in the vicinity of EHV transmission lines. For long lines (utility) this actually represents a small amount of power loss and inefficiency. But even on relatively short lines in an aircraft, this effect can produce a considerable amount of RF interference as well as ozone, which can affect some materials adversely.
