If you've only got third party insurance, there's no point.
If you've got third party, fire & theft cover you'll have to consider what the chances are of it being stolen or torched, how much you'd lose if it happened and how much you'd get paid out after the excess on the policy. For a car like mine, bought two years ago for �350, parked in a low-crime area, it would probably be pointless to insure it. If it was a Ferrari parked in Peckham it would be essential to insure it!
Similar considerations apply to comprehensive insurance.
Chris