Legacy Aircraft as FSX AI or Multiplayer

Applicable
MS Flight
FSXA
FSX
FS2004
FS2002

Many legacy aircraft can be used easily in FSX as user aircraft, but do not show up as AI aircraft or in the role as an aircraft representing another player in a mulitplayer session.
Still, the aircraft label and effects, including noise, are produced.
Overall, everything is there, except from the visual model. It's a sort of ghost.

This problem can be circumvented by deselecting "Aircraft casts ground shadow" in the Options/Display/Aircraft tab.
Though this is also a slight disadvantge with regard to realism, it makes the other aircraft visible.