It's a simple addition, since it's already recorded in the kill readout, what weapons are doing the killing, and it is not game breaking, nor would it mess with mechanics. It would just be another info area, sort of like kill count/assists, and it really is just for the personal satisfaction of knowing. Much like I would be pleased if it kept track of my longest kill streak. It's just data that would make the game that much more nice for me, and potentially lots of other people who appreciate such data.
Further down the line if they did want to mess with mechanics, this could be attached to awards sorta like swbf 2 had. 30 headshots in a row with the rail gun could give a sniper rail buff. But beyond this, it could be used to tell devs how much each weapon gets used, and the weapons that are the most unpopular, would be the ones that get focused on for improvement first. It could also give your hero/class a secondary rank, publicly or privately displayed based off of what weapon you used. Like someone who uses needlers the most, and gets the majority of kills with them, would be needle master
Unnecessary? Sure it is, but then again it's a fantasy war game...putting some more fantasy into it wouldn't hurt.
@betterthanUnoob Steam still doesn't have 8v8. The most it has is 5v5, (10 player rooms)