For a start I very much doubt the insurers will even check. Secondly, if there was no reason why the car should not have a valid MOT that will be taken into account. Thirdly, many assessors will not even check the existence of an MOT. They look at the car as a whole, any pre existing damage, tyres, windscreen etc. I've done a number of these and the MOT issue has never been raised.
I also disagree with the fact that a car without an MOT is worthless. it's not - as long as it is in good nick and likely to pass an MOT (and expert evidence can be given on that).
If I had a 3 year old Mercedes and I had missed the date for the MOT, are you seriously suggesting that instead of say £17k, the insurers would pay out next to nothing? They wouldn't. They would pay book value for a car of its age, mileage and general condition.