It introduces a game mechanic that some people will not like. True! However, with appropriate Unit AI, this would not be dependent on the players ability to micro, but rather the players ability to protect their supply lines.
If this ammo concept for Javelins and Archers was introduced, and you are the kind of person that doesn't want to protect his supply lines, you could quite easily get by with weapon switching, and also things like "scavenging" the battlefield.
I would be in favor of not implementing such things officially until the micro was sufficiently reduced that this mechanic introduces new strategy without increasing the required Actions per minute to be an effective player.