While points are imperfect, I think it does a decent job of showing how well you are assisting your team already.
I believe that there is a technical issue with your suggestion as well. With points, the action is evaluated and awarded right away, so the only data point needing to be saved is your total score. Even with proximity points awarded for being close it woulldn't need to store and query a bunch of data. On kill, it would just award points to the killer, then ask if teammates are in range, if so, give them points too.
With an assist feature, especially where you might use "enemy killed within x second of being hit by a player", you would have a lot more data points needing to be saved and queried as well. With 120 players on the server, instead of simply updating the saved points/kills stat, it would need to also query to find out whether anyone else did any qualifying actions within a timeframe and that happens x 120. It would use up resources for something that the points system is covering nicely I think.
As for "kill whoring", there are a number of ways to interpret what you are trying to prevent.
Strength whoring for 1 hit kills? - Assist numbers is not going to convince someone to take less.
Killing a weaker opponent when given a choice? - When going up against 2 people, it is always a better strategy to take one out first, than to distribute attacks between them both. It is also better to take out one as fast as possible. That will always be the weaker one as even a weak player can disrupt attacks or kill you with a dozen small lashes while you ignore them. Assist numbers isn't going to convince someone that to do different is a better strategy.
A horse isn't considered a weapon and it also isn't you, so aren't getting points for it. Perhaps the dev's could add that in, but then again they may not want to encourage horse bumping in the game anymore than it is already used.