a thief that has seen your seat post before and has planned a way to get it. Thieves carry whatever it is they need to steal what they want, and if it's a seat post with ball bearings in it, they might have the tools to get the job done.

I agree with you that they will get it if they want it and that all we can do is slow them down, but I have simply not been convinced that the bearings are a good enough idea to go out and do it.