It's standard netiquette not to post someone elses plate and it's a good idea not to post your own. Someone could clone your plate, they could hold a grudge against you and when they see your car they could damage it. So many reasons not to post your own plate or that of others.
As for driving around with your plate uncovered that's a no brainer. Firstly you have to and secondly it doesn't go onto the internet.
I think that's fine. If someone is comfortable with their plate being on a site and they post it there themselves then that's fine. It's their choice. But people should have a choice. I've posted a picture of my 131 with it's reg clearly visible but only because the car went to the wreckers 7 years ago so it's of little use to each other. But to post the plate of some complete stranger is not really acceptable.
Hah. I read a thread further down and he's been told not to do this before......