I seem to remember that it
had more to do with the fail-over not working well on the chartplotter

Yes, I've seem the same sort of thing. More than the chartplotter will
probably 'favor' the SeaTalk-connected one first and the the NMEA unit. The
only trouble with keeping one as a spare is corrosion of the open
connections. You don't want to just unplug it and leave the ends hanging.
Be sure to cap them.