The M16 in this photo, which was taken in Afghanistan a few days ago, has a carry handle mounted on the 9 o’clock handguard rail.
I had not seen a carry handle mounted there before so I asked about it on MP.net. BumpkinBoy and scrybe suggested he has it there in case his optics fail. If it failed he would need to detach the scope and carry handle, then reattach the carry handle.
I suppose it is not a bad idea if you cannot afford, are not allowed, or do not have backup iron sights (BUIS) on hand.