In 1999, I lost (left) all my cards, cash, and passport on a train from Budapest to Prague... so I had a new passport issued from the US Embassy in Prague... and what an ordeal that was.
Anyway, the passport is very odd. It has the passport number spelled out in holes (dots) that go through the cover, and first six pages. I had to write in the number myself on the inside. Then my photo is not laminated, just pasted in its respective place with one drop of glue. You can lift the photo on all sides.
It was only valid for two years, but I had it extended and it expires in 2009, and you have to go to page 24 to see that it was extended. My passport lacks the markings to be scanned and so data has to be entered manually.
So, I am always asked lots of questions when I travel and several immigration agents, and gate agents have suggested that I get a new one. On occasion, I've asked why I should get a new one, and in each of those instances, the reasons were generally that it makes it convenient for them.
Given that it cost money to replace the passport, I see no reason to replace it until it expires as it is perfectly valid. Additionally, I only have one page left before no more stamps can be put on, I'm on a mission to fill it up this year!
Thanks to an experienced gate agent in Amsterdam, I did learn a trick a few years ago to help agents read my passport. I have an old visa for China, and agents can scan that and it returns all the same information that the normal passport generates.