It may indeed have been said to avoid giving a warantee but, in my opinion, a car warantee is next to worthless anyway. If you buy a car from a dealer and it's not up to the standard that you or the law expects, you first try to sort it out with the dealer. All the worst dealers will do in these circumstances is do a poor quality repair and take a week or more to do it just so you don't rush back again.
If you are not spending a lot on the car, you should not expect a lot and hopefully you'll get more than you expect. I certainly did with my current car.
An MOT is just a safety check though. A car with a nearly worn out gearbox, clutch and back axle, for example, will pass an MOT if there are no other problems. Having said that, if the car drives nicely and doesn't make any odd noises, the chances are that it will run ok.
If you are uncomfortable about the "spares or repair" on the contract, if you've not yet signed, you can simply cross it out and explain why you are doing this. In my opinion that doesn't make a great difference to your rights but I doubt that the garage would want to miss a sale over those three words.