Laird Al and all,
Yet again it seems I need to remind you of the difference between a warranty and a guarantee.
A warranty is statement to the effect that the goods in question were in a particular condition at a particular time. In retails sales this statement is implied and underpinned by the Sale of Goods Act (SoGA) and are your statutory rights. By virtue of that you cannot extend a warranty. Nor is there a legal warranty period. SoGA will support claims for breach of warranty up to the normal life time of the product or 6 years. The warranty liability rests solely with the person or organisation that sells the product to you. Not the manufacturer (unless they sell direct).
What you have is a manufactures guarantee - which is a written statement of how the manufacture will respond if their product fails within a specified time. As it is not a legal requirement the manufacture can apply whatever conditions they see fit. It is a gift.
An extended guarantee is nothing more than an insurance policy, and like the manufacturers guarantee it can have whatever conditions applied to the policy owner sees fit. However as it is something you purchase you could have a claim against the policy owner if they act unfairly or the terms are anti-competitive.
The choice of whether to use an Ext Guarantee of not is personal, and only you can make the value decision on the chance that the vehicle will need an expensive repair or not. Do bear in mind that the EG's are often limited in their scope, and may not cover some of the more common failures.
I personally no longer take out extended guarantees, and so far I have saved money as a result.