Maintenance contracts are also quite popular since they can charge more for them and amortize the cost over lots of sales. Extended warranties are just one part of this, and often result in the customer paying for something they don't need (how many people have warranties and choose to replace instead of use the warranty?).

The best bet, IMO, is to get advice from someone who doesn't directly benefit from a bad product, but is still an expert. For example, don't take the advice of a salesman, but instead a repair guy friend.