in fact my friend 'Dave' bought (twice) MB-branded NGK plugs, with the MB three-point star stamped on them, knowing very well that this star will only be seen by the mechanic fitting them, and then again four years later by the mechanic removing them...
Totally agree with the principle and I have never bought dealer supplied plugs in my life, however there may be an exception with some MB engines. Specifically the M276 which requires indexed plugs i.e. with the plug gap consistently pointing in the same direction. Genuine MB plugs will index to the same position when correctly torqued and that cannot be guaranteed with other plugs.
Spark Plug Indexing on ML 350 M276 engine