Re: Is that legal now?
Ah, so it is a rule that predates grounded receptacles. That's presumably why server rooms are exempt, along with anywhere else that's up to modern code.
The business I own has three phase power, and the panels are wired in sequence on each side - i.e. from top to bottom breakers use phase 1, 2, 3, 1, 2, 3, etc. (standard US 120v, along with a few things that use "two phase" 208v, and rooftop HVAC units that use three phase 208v) so I've got multiple phases in the same room all over. I have three meters for different sections of the building, and each meter is fed by a separate utility phase.
Since the main/middle section has almost outgrown the two panels that serve it, some later additions were wired from the single panels in adjacent sections. Once in a while the utility will have an outage that drops only one phase, so you get this seemingly random mismash of things that lose power and things that don't - i.e. one 'single panel' section will lose all power, plus a few things here and there in the main section, or most but not all of the main section will lose power. The first time it happened I had no clue what was going on, I thought the building was possessed! :)