The bungees on a hitch rack are just to pin the bikes to each other to keep them from swaying, you can also use the 4000 lb test strap that most come with instead, but most people find bungees to be easier and more versatile. I'm talking big honking trucker-type bungees here, not some stuff you buy at the dollar store.

I also prefer a hitch rack because I can tell in 2 seconds of visual inspection whether everything is battened down and ready to roll, versus a roof rack where any number of things can go wrong and it's hard to see and test everything at once.

Also, the cops probably see more bikes come off of hitch/rear racks because there are more of them out there, and the really cheap strap-on models tend to be the choice of people who don't really know what they're doing. You'd have to work pretty hard to get the bikes off of a hitch rack or a Saris bones or something like that. Especially with multiple bikes on there strapped together, the attachment of the bikes is redundant 9 ways from tuesday, where as with a roof rack if any one point of attachment goes you're screwed.