I have a bench supply which I can use to power a breadboard at 5 V. Is it a bad idea to power the the Arduino by going directly to the 5 V pin? It sounds like both this method, and the USB are unregulated, so there would be no difference. Is it safe? Are there any caveats to this?
5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
They say they "don't advise it", but why?