It's pretty darn accurate, IMHO. Many will argue the only true indication for a change is the mileage on the oil. What this sensor does is measure the capacitance of the oil. The oil capacitance is a known value when new, and the sensor can also detect a good quality oil from a poor quality oil, and takes this into account as well. As the oil ages, it becomes contaminated over time, mostly by-products of combustion, but also moisture (condensation), and fine metal particles (think of more like a slurry than actual chips or shavings, just product of normal wear). This changes the capacitance of the oil, and the oil quality sensor monitors this.
The actual FSS system, which is a component of the instrument cluster, also observes the actual time the oil is in there (as reset with the FSS) and mileage. It takes all this into account, as well as the start/stop cycles, speed, idling time, etc to calculate this out. The system does calculate very accurately when the change should be done. I don't believe they are pushing the change out to any great extreme, I believe they are actually giving a pretty safe margin.
One thing to also keep in mind when analyzing this is that the engine is holding a LOT more oil than what the "3000 mile club" is normally remebering from their 3000 mile youth: This is 8 quarts compared to the 5 quarts the "3000 mile club" grew up with. Also keep in mind this is (supposed to be) synthetic oil. A higher grade of oil (such as Mobil1) should be able to do a good job of lubricating the engine for more miles than a conventional (AKA Dino) oil.
But yes, if you change the oil yourself at a mid-point in the oil life, the oil quality sensor will sense the fresh oil and do a recalculation on the oil change. The system can even recognize if oil was added between changes, just by level monitoring, and also quality monitoring, ie the change will be delayed even by adding 1 quart(not that you shouldn't add a quart if it needs it). Hard to get anything "past" the FSS system.
If you really dislike this system, I would still encourage you to go with what is called for by the FSS during warranty, then change to however YOU want to do it afterwards.
What is really the likelyhood that you will really be driving this car in 15 or 20 years at 500,000 miles? Not too likely. You are better off (IMHO only!) to go by the book, keep good records, and when you are ready to sell the car, you have the records to show the prospective new owner and tell them it was kept up strictly to MB requirements.
(Please, I don't intend to start yet another oil war!)
PS You won't really be actually "missing" a free FSS service per se, but the mileage will be higher than it normally would be at the next FSS service than it would have been if you left it alone.