It depends on how much it is and how much it covers.
For example, at my store, for those speakers it would be about $75 (per speaker) for a 10 year coverage plan, that covers everything except physical damage. Since Polk covers them for 5 years, you would basically be doubling the life of your warranty. Also, if you were to have problems with them, you just return them to the store as opposed to sending them to a Polk authorized service center.
So it is up to you. Most people call the warranties a rip-off, because the stores make a lot of money on them, which is true, but I wouldn't classify it as a rip-off. The reason the store makes a lot of money on it is because there is a very good chance you will never have to use the warranty. But, if you were to have a problem with the speakers, the store would fix or replace them, no questions asked. I would consider it a rip-off if you paid for the warranty and then the store didn't honor it.
Just recently I had a customer call me up because her $5000 Sony LCD crapped out after a year. Something was shorting inside and was causing the lamps to burn out after a week or so. So we sent someone out to her house and we ended up bringing the tv in and fixing it. Since her Sony warranty was expired, had she not had the warranty, she would have been spending a lot more than the $500 she paid for it getting her tv fixed.
Since you got the speakers at such a discount, you might want to get it, simply because if your speakers take a dump, you might not be getting that discount again, and you would be SOL. But, like I said, up to you!
~Chuck