what's to stop people from exploiting that?.
You can never stop exploitation of any rating system. The key is to design any ranking system such that exploitive behaviour, does not hurt the team and preferably actually helps the team forward.
Which is my primary issue with using K/D ratios as a base for rankings, as the exploitive behaviour is to play very secure and defensive which stops people from advancing, which can already be an issue right now.
With a kills/hour ratio when people exploit and maximize it people will try to kill more people in a shorter time span. Overall this helps the team. And in case of fear of ramboing a combination of K/D and K/H ratio could be used.
Another is capping, in RO1 a lot of people cap a zone and then immediately leave the capzone without properly defending it. So a reward system for capturing should not only reward capping but defending as well.
-----------------
The key for me is for ranking systems to include the most obvious exploitive matters and add that in the ranking system.
With K/D ratios the easiest way to exploit is making 1 kill every 5 minutes, without taking any risks. Add in a factor that looks at how many kills someone makes in a minute and that exploit won't work any more.
With capping the easiest way to gain loads of points is by capping and quickly going to the next cap zone. If people get rewarded for defending a cap-zone, then there could be much less of an reason to leave a cap-zone undefended.