This is common to many items, that we are pressured into purchasing things that we just don't need. But Extended Car Warranty schemes do seem to be worse than many (although I was once offered a warranty on a toaster which cost more than hte toaster itself. Bizarre.)
For one they're expensive and secondly, they may well mean you're paying for something you're already covered for.
It's very much worthwhile having a look at this site on Extended Car Warranty schemes, as they lay out the whole process, what you might indeed want one for and what you might not.
In short, they explain it all, when and where it's a scam and when and where it is not. Extremely useful information in fact.