Garmin's implementation of RS-232,RS-485, and CANBus is dependent upon good shield terminations (drains) which the Garmin connectors have in spades. Further, the connectors (not the back shells) are MilSpec TE/Amphenol parts (read: $$$) and machined D20 contacts (read: more $$$ plugs & sockets).
So, yeah, can save a few bucks on cheap stamped tin D-shells and not capable back shells and maybe spend a ton of time down the road debugging problems.
Or buy the genuine article and not worry so much about it...