Actually, new Mercedes engines all have platinum plugs and these engines run hotter than previous engines. However, it IS true that platinum plugs should not be used in engines prior to the current 3-valve engines. The older engines were designed to operate best with Bosch copper plugs.
Platinum plugs are designed to last longer not because of the use of platinum but because the tip is protected. The result is that they are not as effecient (at least in older engines) as a normal copper plug.
The recommendation is correct, no platinum plugs prior to the current engine line.