but if the gearbox drops out or the clutch explodes then it would be all worth it then wouldn't it
Thats the nail hit on the head.
The word 'warranty' leads most people to believe that everything is covered, mainly because it is a word associated most with new cars where most things ARE actually covered.
Think of it more as an insurance policy on the major parts of your used car - it will probably cover a lot less than you expect it to but its a safeguard against big stuff going bang.
You take out house insurance knowing only certain things are covered, you probably pay out mobile phone insurance, boiler cover etc etc
Against the cost of your car, the warranty is a small percentage and its classic 'buyer beware'. Make as sure as you can that the car is actually a good 'un before you buy it, then have the warranty in place as a safeguard. You may have claims turned down, but even one approved will probably repay you your premium in a bill saved.
Even on new cars, warranty isnt as comprehensive as you would like. Its European law that every new car must have a 2 year factory warranty, its the UK importer who pays for our 3rd 'bonus' year which is why the cover reduces in year 3. Even in years 1 and 2 some things stop being covered after certain mileages and time limits - they become adjustments and wear & tear therefore chargeable. My Leon has 12k done in a year but I know my noisy brakes wont be covered, nor will my wiper blades and washer jets.