Most of the time I carry the bike inside my 4Runner. When I need to carry additional bikes I have a
Thule 916 XTR hitch rack. The bike trays have built-in cable locks. Like you say, it's not totally secure but it's enough of a deterrent where (hopefully) nobody will swipe the bike while I'm having lunch/beer after a ride. It hasn't happened yet. I see you're in Irvine so you probably ride all the same trails I do (Whiting, Luge, Aliso, Crystal Cove, Santiago Oaks, etc.). Never had a problem in this area.
I never, EVER leave bikes mounted to it overnight and/or anywhere unattended for extended periods.