Even the very best bike rack security merely slows down a decent bike thief...
Security is an interesting thing in terms of human behaviour H(aich)aakon :wink:
I find the most practical is a good quality bicycle security cable and pad lock. Looped through the bicycle frames, through the carrier lanyard point and pivot base and then secured to the vehicle chassis. Locks the bikes to the carrier and then the carrier to the car. This is a highly effective solution because it discourages the opportunistic thief and does not tempt the honest man.
Covert or subdued security does not discourage a thief until after they have attempted theft and then it's too late.
If you go overboard, you present a challenge to a would-be vandal and they may turn around and lash back and cause damage just to teach you a lesson. There are sickos out there.
After all these years of simple and visible security, I have never had a problem - even overnight in Halls Creek. The only thing is that you should never drive with the security cable attached. If anything is going to snag a branch or rock it'll be the cable.
In a nutshell, keep reasonable to not tempt a thief but if they must steal your bike, then they will but make it so that they don't cause thousands of dollars additional damage in doing so.