Misfuelings are a big concern for me. Not many people own diesel cars in the USA and the damage done by running a diesel fuel system on gasoline or gasoline-diesel mix is too costly not simply to lock everyone out.
I was faced with this issue when a kid at the Ram dealership filled up my diesel truck tank with gasoline. I ended up getting a brand new truck out of the deal, but still, it was a long harrowing experience.
There is also someone (or someones) that have been following me around doing damage to my vehicles. Somebody bashed a dent in my front passenger door on the GSW and two dents in the driver door of my pickup. Also, after parking my camping rig on my own private property over night, someone pulled the link pin out of my towing receiver, I can only guess in the attempt to add instability to the rig. I found it when I was backing into a different location and the trailer was acting really strangely.
I am paranoid, and prefer to keep the fuel tanks locked up...simply because the cost of replacement is so high. I would have liked to have had my DEF tank locked up from the beginning, but I just figured that the car could operate without that if someone had damaged the SCR system with contaminants in the meantime. Fortunately, it didn't happen.
I talked to the folks over at the BEM Auto factory (they're local to me) and I've heard some stories now about people who've actually had their DEF systems contaminated (not on the TDIs) either by well-meaning negligence or by malice. I heard that one gal from a towing company called up and ordered locking caps for all of her Ram tow trucks after the costly replacement of one trucks SCR system. Apparently vandals had put some soluble material, likely sugar, into the DEF tank, probably thinking it was the fuel tank as the DEF fill neck on the 2-ton+ Ram trucks sits at about where the fuel filler neck is on a regular truck. Another story related was that a field mechanic's DEF systems on his Ram trucks had been ruined by techs unfamiliar with diesel trucks filling the DEF tanks with diesel.
From what they told me, the only reports of fuel actually being stolen come from "hot-shotters"...or guys who do commercial towing/hauling with 1-ton diesel trucks. All the other reports come from people who've either had their vehicles misfueled or tampered with, or who want to prevent that from occurring.