Both of the mapping heavyweights, TeleAtlas and NavTech are pretty much on the ball when it comes to updates.
The maps are "Amended" with information such as proposed roads at the time of software updates, but obviously this is never going to be 100% precise.
Roads and Bypasses are changed on a daily basis, and maps are launched once or maybe twice a year depending on supplier.
Root around in the car, and you will find a perfectly useable AA road atlas, and its probably more than 5 years out of date, but the point I mam trying to make is that you would still consider using it to get from A to B.
Personally, I update mine every year in my VW, and yes, I do see a benefit from it, but then I cover a phenominal mileage every year, so the small additions really do add up in my case. That said, the Nav disc in my Ferrari is probably two years old, and even that doesn`t miss a road - if it does, then I haven`t noticed it as it doesn`t really do the mileage to warrant yet another hundred quid for a disc.
Horses for Courses really - how often does the nav mapping fail to get you where you wanted to go?
:-/