[Dr43%er]

I know it sounds petty, but as has been mentioned, most cars that are untaxed will not have an MOT and the driver as a rule will be uninsured. If these people have crash into your car it will cost you, or your insurance company money. The insurance company will get there money back one way or another, so expect your premium to go up. Lets say it goes up £30. That is £30 out of your pocket.
If someone robbed you of £30 would you expect the police to do something or would you class it as wasting police time when they could be out there catching real criminals?

With regards to forgetting to tax the car and getting someone in trouble. Yes it can happen. I know, I have done it. I got pulled over while they checked me out. As I had insurance and a valid MOT I just got a fine. I paid it with out argument as forgetting is not an excuse. In fact I was glad to find out I was not taxed as if if it had carried on being unnoticed then I could have got a £1000 fine and my car removed from the road. If had been involved in a crash then there is the possibility that my insurance company my not have paid out. That could have left me many £1000's of pounds out of pocket.

I know I would rather be told of my error. I think most people who have genuinely forgot would.

Those who are not taxed due to not having an MOT or insurance (for those that don't drive, you have to have those documents to get tax) then I doubt they ever had any intention of getting tax, so i would not feel sorry for them getting in trouble, as if they hit my car an I was not there they would drive off. No question. So it would cost me money. So **** um.