Originally Posted by Herzlos

You're talking about the ones that clip into the edge of the open tailgate and then are held in place with straps?
If so, they are going to be pretty hard to steal without breaking in the car first or damaging the straps, and they aren't really expensive new so unlikely to be worth much used even without damage. The bigger risk is that someone may try and cut the straps to steal the bike, if the bike is locked onto the rack.

If it's a towball mounted one, Thule do a barrel lock for about $20 that'll prevent it being removed, which is worth it because they would otherwise just screw off. To be honest I don't know why they don't already come with it fitted as most other brands do.