The diagnostic socket ;cover' grounds pin 18 which is the diagnostic serial port input. When you use the diagnostic cable from PC to ECU the diagnostic cable ECU end plug has a 400 ohm resistor from the ECU input port to ground. So I think that there was concern to reduce any electrical noise at the input to the ECU. I assume that noise on the diagnostic input mat cause the ECU to think there is data at the input, which could cause it to spend processing time, and maybe if the ECU processor gets stressed it is apt to cause glitches.
Many years ago I saw a reference stating that there was an internal Rover document with a detailed technical description of the 14CUX, I think it was on controlled circulation in the Rover group and I haven't seen a copy out in the wild. Most of the detailed information tends to be from reverse engineering by the TVR guys.
