I prefer to lock the tailgate from being opened as well.
I know that tailgates do get stolen but in the nearly 60 years my family, friends and I have owned trucks I've never personally had, or know of, one being stolen.
Unless you count the time or two the entire truck has been stolen...
I would agree. If you are an experienced car thief you could drive off in most any vehicle and strangers would not raise an eyebrow.
But struggling with and then walking away with a tailgate is not exactly a 'snatch & grab' unless you are very well hidden.