Having it be a grid or axis is going to both incentivise you to game the the numbers in one direction, or pigeonhole you down one area of morality.
That is only the case if they make the entire system transparent.
Such a system works better if it's hidden. You don't get to see what dialogue options provide what results. You don't get to see anyone's reputation chart. You only get your intuition of what you think dialogue options represent and you only know a characters feelings about you through how they interact with you - Just like in real life (Where yes, you can "Game the numbers" by trying to act a certain way with someone, you just don't know exactly how everything will play out beforehand and things get very complicated when you interact with such a person amid OTHERS whom you don't act that way with)
Sure, even with a hidden system, on multiple playthroughs and with eventual compiling of data from other players, you'd eventually be able to metagame certain outcomes - Though how bad that is, given that roleplaying a particular character will tend to involve some level of metagaming (Since you'll already know how you'll respond to certain events and characters beforehand) is subject to debate. Certainly, it doesn't diminish the value of implementing a better system, especially given that the current systems are also very easy to metagame (Even more so given their lack of depth)
