What happens if you forget to unplug the tool before working on the switch? Both conductors on the upstream side of the switch are hot. There is obviously nothing wrong with opening both conductors, but I have always felt this was a poor reason to worry about it, because there will always be live conductors present in virtually every system once you open a secured cover without disabling power. The neutral is still a current carrying conductor too. Anyone foolish enough to open the peckerhead of a motor without cutting power deserves to have his widow selling his tools to the rest of us on eBay.
I am not saying it is a bad idea or anything, but simply that people should be told that it is their option if they want to replace a single-pole switch with a double-pole switch when they make this conversion. It is not mandatory that they use a 2-pole switch on a 240 volt motor. (I just read this within the last 24 hours, but I cannot remember which article, but I am sure it was within NEC 430.)
Oh, I found it!
Article 430.84 Need Not Open All Conductors. The controller shall not be required to open all conductors to the motor.
Exception: Where the controller serves also as a disconnecting means, it shall open all ungrounded conductors to the motor as provided in 430.111.