They date them for a reason. I can see the problem with belts because of the material and webbing breaking down, but can't see it with the seats, especially since they're made out of carbon fiber.
Maybe it has something to do with the current safety standards. Perhaps they date it in case you want to use the seat in 10 years and even though it says FIA certified, it wouldn't be up to todays standards?
The only problem that I could see is if you join a club or do a DE that is a stickler for safety and checks dates. If they check dates, they might not let it pass inspection.