There are a number of reasons why people hide their number plates. Generally I try and blank mine out for the security of my car, as there have been a lot of cases of cloning reported recently. I also have a private plate, and plate theft is on the up. Not forgetting the car/tortoise theft issue raised above. From my point of view blanking the plate is damage limitation as you can't stop people taking pictures at runs and events, then posting them up unedited.
Then you have the more general stuff. Anyone who listens to the news will have heard several warnings recently about being cautious about posting private/personal info on the net. Trade unions and recruitment agencies warning that employers are using searches of forums, and facebook type pages as part of their shortlisting of candidates for jobs. Some people post their whole lives on the pages of forums when chatting with friends but forget it is all in public view. Imagine what a house breaker or car thief could do if you post stuff like "Here's a picture of my brand new £30k
MCS outside our house, how do you like the new private plate?"....then a few days later, "Looking forward to my two week Jamaican holiday which starts tomorrow..." Easy pickings!!!
Then of course there is this
It's not about cars it's about protecting your identity. Post away, join in the community, but just be careful what private info you give out. I have seen addresses, phone numbers and even bank account numbers on forums before.