Mission scoring should ONLY stay if it is not required for 100% game completion or for any external rewards (achievements and trophies). I personally loathe the idea of having "scores" in any form in a sandbox game, particularly GTA. It's not a game about competition or doing things in a particular way. The more it gets dictated, the less freedom you have.
The only way I'd really wish to see mission scoring remain in future games is if the only thing achieving perfect scores could potentially do is reward you with in-game stuff (weapon spawns at safe house, etc, something like that, but only convenience stuff, nothing that would near-impossible to acquire in-game otherwise).
That said though, probably would be better IMO to keep it out all together.