I actually thought of making the unexpected salvo a passive that activates on the first volley of arrows fired by the Rangers when stealthed in the woods. That'd be really neat.
Regarding your point, logically speaking you are correct. However, it would bring balancing-complications I think. They would lose their potential towards damaging buildings and it would be necessary to tweak fire arrow princing and costs etc..
There are more reasons that could be given, but I think some more diversification could always take place, but it is not the priority now. Actually, you gave me a good idea, thanks for that!