Boosters are being redesigned each year to make them safer. Booster seats today do a better job at positioning the lap/shoulder belt on the child than many booster seats made 8 years ago. Also many seats have added improved side-impact protection.
Just look at the new IIHS booster rating article :
http://www.iihs.org/news/rss/pr090810.html
It's clear that some manufacturers are taking the ratings to heart. Harmony Juvenile Products has 5 BEST BET boosters, more than any other manufacturer. One of them, the Harmony Secure Comfort Deluxe backless, wasn't recommended last year. The company modified it to eliminate the earlier problem with lap belt fit.
From the IIHS ratings news release, it is clear that some manufacturers are trying to improve their seats. With booster seat expiration, those older, less safe booster seats can get removed from use. Without seat expiration, shield boosters might still be on the market. They were never recalled, but proved dangerous, so manufacturers stopped making them.
Booster seats do suffer from plastic degradation as the shell still needs to be strong enough to hold the seatbelt in place during a crash. Lost parts and instructions also are something that happen to older booster seats.
Booster seat expiration is probably a little less of an issue than with a harnessed carseat, but it is still something that should be followed.