Then why not just put "instrumental" to the lyrics field of all of your instrumental songs instead?
Mainly because this breaks the Lyrics? function, returning "Y [unsynched]" instead of "N [no lyrics]" (as "instrumental" is not actually a lyric, just another way of saying the the track has "No lyrics")
Also, an instrumental is any musical composition or recording, primarily without lyrics or singing.
It can be from any genre; classical, rock, avant-garde, baroque, pop, etc, etc, etc....