I did that on both of our cars, got a 3 year deal when they were discounted about 2 1/2 years ago.I had a 3 year service plan on my previous car which was worthwhile.
The secret is to take it out before the B service is due to get the main benefits. Doing it this way ensures you have a B/A/B service.
If your car is 18months old, it should have had an A service at 12 months so your next service is the bigger B service.
Taking out a service plan does mean all filters/oils/plugs/brake fluid/atf changes are paid for within the plan. Consumables like discs and pads are unfortunately not covered.
Not sure how long your warranty is, but keeping main dealer service history also gives you Mobilo accident and breakdown cover for 12months.
That is a very confident statement you have made.Brake fluid change is not an optional item. if its on the service schedule it needs to be done - end off.
Going back to your original statement, no it does not routinely "need" to be done every 2 years.
Mercedes as part of their Service Care plan say additional items such as brake fluid and screen wash are included in the plan.
This is a controversial subject and most people will have their own opinion on it, but most people will simply have it changed because it is classed as a safety item.