Then again, some songs (e.g., a lot of contemporary American pop and rock "songs") consist as much of instrumental (ncluding percussive) riffs as they do lyrics and melody. If sung a cappella, sans percussion, there would be fairly glaring gaps that would probably be tedious.

But I would submit that the human voice can and often does substitute for instrumental parts, including percussion. A cappella groups can be very impressive in that regard.

I do understand, though, that some traditions call for instruments only or voice only.