I'm going to attempt to speak on Larian's behalf here, so just bear in mind this is my interpretation.
The existing system has 3 stats for 3 styles of combat
Strength -> Melee
Dexterity -> Ranged
Intelligence -> Magic
plus 3 stats of "Nice to have" for every class.
So while you could roll to hit into Dexterity or perceive stats into Perception the splits exist so every style of combat has its basic damage and some out of combat perk, but no more. The kickstarter post specifically states that this is not based on pure realism but an attempt to make balanced stats.
You've got that wrong. It's not one key and three "nice to have" stats.
A warrior who has a lot of strength but can't take a hit or make a hit is worthless. If you pump up both his CON and STR so he can take a hit, and do a lot of damage, but can't hit anything because of poor PER - he's also ineffective.
Meanwhile, a Wizard really does have one "must-have" stat for all his abilities and a bunch of "nice to have"'s. A ranger is in the middle, with 2 must-haves.
The problem is that it isn't balanced.
