I guess it has something to do with people being able to access some of your personal files from the license number, at least that is what I have always thought. I don't cover mine, I have no money to take.
In California, ever since that actress was killed by a stalker, it's been illegal for DMV to release info based on a license plate. Before that, $10 and you could get the info.
I used to not cover mine- then everyone I know convinced me I was asking for trouble.
I guess in my case I enjoy writing the occasional detailed 'build' or 'progress' report. I like to put in as many details as possible but have been hesitant to include prices of things. I guess if someone read I just dropped $$$$ for a certain new part I'd be afraid that'd be inviting someone to look up my address and steal it or the whole car.
I'm really paranoid though...
I don't worry about it too much on the streets because they don't know I just spent 6 months doing/ building this or that...
Also if I take pictures in my garage I wouldn't want someone to see some parts or tools in the background and pay me a visit some night...