Originally, in 99 and 98, they were recommending 4 years, 100,000 miles.
Then, they came out with this Maintenance Commitment Plan where mercedes pays for the FSS Services during the warranty period.
There is a little confusion concerning the FSS items that are affected by time.
Many techs assumed that the "born on" date on the door jamb would be the proper reference point for things like brake fluid, coolant and spark plugs for example. Some say No, the delivery date is the proper date to use, but it may be difficult determining the true delivery date, but the production date (month and year) is on this label in the door jamb.
SO, the thought I have in regards to changing the spark plug recommendation to 5 years is this:
Say a car was delivered in lets say July 2000, but it was built in January 2000, (car sat on the lot for awhile before being sold).
Car comes in, under 50,000 miles, in May of 2004, original owner, still under warranty, for an FSS service, Maintenance Commitment Plan, MB paying for it, right?
OK, tech is checking the "born on" date in the door jamb, hmmmm, car is over 4 years old, Spark Plug time!
I believe MB changed it to 5 years so there's no way the car will be eligible for free spark plug change, not unless the car sat on the lot for at least a year before being sold. (MB doesn't want to pay for those 12 or 16 spark plugs any more than you do)
I really though the interval was changed to 5 years in 2001, I could be wrong, as you seem to indicate.
But 98 and 99 it was 4 year time interval on plugs, I still believe this is better than waiting the extra year before replacing the plugs. First of all I'd worry about the plugs getting seized into the holes, and secondly, assuming the vehicle isn't constantly being stored, then it's being used in-town alot. Then you don't worry so much about the MILES being travelled but the amount of time (HOURS) the engine is running, in other words slow speed driving and idle time at stop lights, etc.
Hope this answers your questions.

Gilly

[ PS added to this post only:
If the vehicle is driven almost daily, such as in-town driving, then the engine is accumulating alot of hours. The engine is being operated a similar amount of HOURS as a car which has high mileage, just not accumulating the MILES as fast ]