the reason why your should be mot'ed at least is - if your car breaks down every 5 minutes, causing a red flag every 5 minutes to tow you back to the pits..everyone is going to be very annoyed at you for wasting their track time. I know an mot isn't a guarentee that its going to stay in one piece, but it stands a better chance than a car without an mot and is a bag of old rusty nails.
Tax and insurance are not important. However..if you take your car uninsured onto a track, it makes you think twice about pushing that hard that you end up falling off the track. But, you do run the risk of some idiot who doesn't care and ends up crashing into you. Take insurance out if you really want, but then if you insure your road car, crash it, how you going to get home? how you going to get to work the next day? etc...