Before I got my own wheelchair, when we would get a chair from the resort, we were given a bike lock and told to lock the wheel on one side so it could be moved but would still be difficult to steal. And I agree that for the most part, it isn't very likely that a wheelchair will be stolen, but it does happen on occasion.
My personal experience with this was on time on our flight to WDW, while waiting for the aisle wheelchair, someone opened up and sat down in my wheelchair which had no arms, legs or seat cushions and which was sitting closed in the jetway and went off with it. Fortunately, my spouse had carried out the wheelchair parts to get it set up for me and saw them. He ran after them and then the person in my wheelchair proceeded to argue with my spouse that it was an airport wheelchair. Even my spouse pointing out that it had no legs, arms or seat cushion didn't change the person's mind.

But the flight attendant did manage to convince them that they had to give back the wheelchair, so no harm was done.
The second experience was at DTD Guest Services. My spouse had gone in to get an error corrected on his ticket and I was waiting back from the desk for him. A woman came racing in, right over to where I was sitting in my wheelchair and said: "I need that chair right now."
I said: "I beg your pardon?"
She said: "You need to get out of it right now so I can take it."
I said: "This is my wheelchair."
She said: "But I need one now."
I said: "You don't appear to understand, this is my personal wheelchair."
She said: "Well, what am I supposed to do? I need a wheelchair now and they told me I could have the first one that was returned."
I said: "I'm not returning this wheelchair because it is my own wheelchair."
She wasn't happy, and upon leaving Guest Services, told the people waiting outside for her that there was a wheelchair but that that woman wouldn't give it up.
I laughed and laughed about this until I had tears rolling down my face.
I have found that for the most part, people aren't going to deliberately take a personal wheelchair. But often people don't pay very much attention to wheelchairs and they seem to look pretty much the same to many people. If someone could head off in a wheelchair with no legs or arms

I wonder how much of an effect personalizing a wheelchair is going to have.