I asked that very question when corresponding with BMW UK and was told that following feedback from customer surveys the majority of owners stated they never used the spare key hence the introduction of the plastic one.
While we are on the subject of surveys apparently it was customer surveys that resulted in the inadequate tool kit we are now blessed with, the majority of owners said they never used the tool kit.
Two metal and one plastic here as well, apparently officially you only get one metal and one plastic but most dealers order you a spare metal one as a matter of course when you order a new bike from them