Naim's recommendation is most likely based on their listening tests, which they, I assume, found most PC's to be a negative. Since they cannot (or will not) endorse one or two units that they may have found beneficial or neutral, they'll take the easy way out and recommend none.
Another factor to consider is that some power supplies do not have oversized transformers and/or filter capacitors in order to get the sonic benefits of low PS output impedance. But these power supplies are more sensitive to current restrictions than beefier ones. Regardless of what the marketing wizards say, all PC's will impede instantaneous current to some degree, lest they found away around the laws of physics.
Another factor to consider is that some power supplies do not have oversized transformers and/or filter capacitors in order to get the sonic benefits of low PS output impedance. But these power supplies are more sensitive to current restrictions than beefier ones. Regardless of what the marketing wizards say, all PC's will impede instantaneous current to some degree, lest they found away around the laws of physics.