Securing bikes while on a car hitch mount
My preference for my hitch rack security is to use a 5 foot Schlage 999478 High Security Chain with Cinch Ring to pass through the frame(s) and use with a long kryptonite evolution U-lock to secure the chain and my bikes to my hitch rack, which is secured to my car with a locking hitch pin. I also will incorporate a separate cable lock to secure my wheels (I just use a loop style and thread it through the ulock as well).

I figure this way, if anyone wants to steal my fancy mountain bike, they at least probably need power tools to easily steal my bike. I prefer this method so I can more safely leave my bikes unattended for short periods while on road trips or during after ride beer stops. 

My opinion is a cable lock is about as good of security as a strong rope. Any yahoo can buy a pair of wire cutters at a dollar store for $3 and steal your bike in a minute or so if you rely only on cables. 

However, the folding plate lock is a bit higher security. I also use one of these on my commuter and would trust it for a medium level of security where I'm leaving the bike unattended for a bit. Maybe not in NYC, but in the smaller city I'm from, I trust it about as much as a U-lock.