I don't think you need to buy an extended warranty. Speakers generally don't go bad on their own, many years ago older models could get driver rot or the foam around the woofer would go bad, but even that would be well after a 5 year warranty passes. I buy used speakers all the time, got a pair of 20 year old Energy speakers in my office right now. Unless you plan to play your speakers at extremely loud levels the speakers will be fine.
Abuse will most likely be the biggest factor to something going wrong with the Pioneers. Clipping or distortion from an underpowered receiver is one way. I've probably owned 50 speakers over the last 30+ years and only one had to be repaired. It was due to someone cranking up the music at a party and it blew a satellite speaker.
Post your question about reliablily in the Pioneer speaker thread to see if anyone has had any problems.