The risk is minimal, anyone who takes the lid of kit whilst 2x different phases is live is asking for trouble. The scenario could (not would) be if equipment power shares using 2x independent PSU's running from different phases. Even in this condition using switch mode psu's the effect is unlikely to cause a problem. But why take the risk, minimal as it maybe. In Broadcast and related systems (editing, post production houses etc) often other non engineering types have access to power connectivity (when they shouldn't) and this is where problems could arise. Maybe it is considered to be overkill and you could be right, but ( I use this word a lot) we take safety very seriously and make sure there is no likelihood of having mixed phases no matter that the problems of doing so is minimal, i say again, why take the risk when we dont need too.