Excellent point. Darn, I crawled all over 404 before answering, but I didn't expect to find the answer under a paragraph titled "Marking"! I believe I stand corrected.
So the only switch (as a motor controller) NOT required to open all conductors would be one that has no OFF label???
In that case, 430.84 would only apply to magnetic or automatic (e.g., pressure) switches, and not to manual switches, which would obviously have to be labeled with an OFF position.
But what about a 240-volt air compressor having a pressure switch that breaks only one conductor? If the On-Off "switch" is just a lever that disables the activation of the pressure switch, doesn't this violate 404.15(B)?
Anyway, back to BigB: Given that (1) both ungrounded conductors have to break, and (2) there are three motors that probably all want to run or stop simultaneously, and (3) those motors are probably upwards of 1 HP anyway, it sure looks like the snap switch wants to be the 120V pilot circuit for a group of contactors, hey?