In over 35 years in my +8 and having toured extensively through Europe, over 100k miles I have never had a problem with bags behind the seat. They have never been touched. I should add I have no roof or sidescreens and just a tonneau cover with central zip. My seats don't recline.
I did loose a suitcase off the back in the late 80s but that was my own silly fault, it wasn't tied down securely enough. Hit a pothole on the Italian Autostrada near Venice, fell off and a Swedish 38 ton truck ran over it. Wasn't much salvageable.
