What's the point of the full move counter in FEN? For what do you need to know the number of full moves? You still can't recreate every move in the game with FEN anyways, so I don't see the point of the full move counter. Is there any rule that depends on it?
You may want to record a fragment of a real game starting from move number 24, for example. Then you specify the FEN after the first 23 moves and set the full move number to 24.