They're helpful if you don"t want to bother with going into a store to get it replaced. It happened to my friend recently (iPhone 3G). Battery died, she went to a store and one said it was the battery and they'll charge 80 for a replacement. They said they would have to order one, so in the meantime she went to an Apple store in another mall. The other Apple store tried to claim it was water damage near the dock but wouldn't show her the water sensor.

Luckily she just went back to the original Apple store and got it replaced.