Yes, I don't bother personally.
If I buy from a main dealer I get a 1 year warranty.
You normally know if you bought a lemon in that time.
I then self-insure against failure by keeping a slush fund - pay the money you would have spent on a warranty into that as well as a bit more for a while.
You can of course be very unlucky but I think with modern Mercs, and certainly the higher order, they are much better built than they were 1994 > 2006 in the Scrimp era.
Regular and timely maintenance / fixing of problems tends to help in avoiding big bills.
Last thing I want is to contact a 3rd party and be told everything isn't covered due to wear and tear - and having to pay for the repair as well as the sodding warranty in the first place.
They might not all be that bad but I don't want to find out the hard way the company I used, is!