The answer is simple:
The main thing to understand is that the ADI-335 meets two different requirements in one instrument. Standby ADI for power failures and standby CDI for display failures in single pilot operations.
The nav function is a way of meeting the backup CDI (nav control/display) requirement for Single pilot IFR. The idea of the CDI is to cover display failures. If the displays on the pilot's side fail or the bus fails the backup CDI will pick up the other (#1) NAV. If the pilot's symbol generator fails he will be able to use the reversionary mode to copy the copilot's displays on his own normal display but will not be able to control the nav through the display controller ....therefore the backup CDI. If you aren't single pilot IFR you don't need either nav on the standby. Baseline prior to a/c 528 doesn't have it.