Ok true those are some valid reasons.
Hmm k for covert ops nvm, mine spotting isn't something I would rank as being skilled or overly useful for objectives.
Now for those graphs I'll give it a go :p
So here we have the original formula with giving 4xp for kills, -4 for deaths and 1 for headshot.
I just looped over kills(1 to 200),deaths (1 to 100) and headshots (1 to 300). See below this picture for further info.


Now I have zoomed a bit. You can see 3 arrows.
The red one : indicating the kills gradually increasing (for every amount of kills I looped over all deaths and for all deaths I looped over all headshots).
Thus making the rate increase.
The green one indicating the deaths increasing thus the rate needs to decrease.
The black one indicating the headshots increasing the rate when you do plenty of those.


Now if we look at the ratio formula:
We don't have those clear relations anymore and the image becomes more messy.


Now I zoomed a lot and what we see is that the headshots are way to dominant.
You don't see any change by the deaths and only a small change with respect to the kills.