Naim have always preferred the DIN plugs as they feel they're better from an electronics perspectine. There's some discussion of this over on the Naim forum, or there was a couple of years ago.
Naim weren't the only ones - Creek originally had DINs, as unsurprisingly did a number of German manufacturers. However, Japan and the US always use RCA, so that became the standard. Some info here: http://en.wikipedia.org/wiki/DIN_plugs.