I think they are trying to avoid scenarios that involve only one path for success.
There is also stealth, a valid path that is neither combat nor diplomacy. Outside of those 3 major options, perhaps specific cases allow you to use particular skills or spells to get by without combat.
I am certain all quests will be solvable through combat in some way... Not sure if the other paths will always be viable but an alternative should exist.
Sigh. I hate games where stealth/diplomacy is a waste and only works half the time whereas shooting people has a 100% success rate.
Look at the trial in NWN2. If you're a fighter, than you just go straight to trial by combat. If you're a rogue who invested points in social skills, then you spend a lot of time faffing about before you end up in the trial by combat anyway, even though the fighter got to skip the stuff he sucked at.
Well there are certain advantages to using stealth prior to combat (scouting, backstabbing/flanking opponents, or simply circumventing them altogether if you can). Diplomacy might give you an advantage (gain allies or avoid combat altogether). But at the end of the day, fighting will solve most situations in a typical RPG quest, so it makes sense to make it the lowest common denominator a majority of the time. It isn't exactly a good thing if stealth or diplomacy always gives you a 100% chance to complete a quest. Player skill gets taken into account during stealth, so if you are not careful in your movements, it might be your own fault you fail. And I know they are trying to avoid "one button win" scenarios with diplomacy, though maybe a few of those will still be in the game when it makes sense.
I sort of remember the trial example you are referring to, but I don't remember the outcomes. From how you describe it, I am sure they will be avoiding such scenarios in this game, though logical outcomes should always trump player desire; sometimes a fight will break out, no matter how skilled you are at sweet talking or sneaking. It all depends on the context of the situation.
The outcomes of that arc were:
You fail the trial in under five seconds, and end up having to do trial by combat instead. This is the option for combat monsters.
You win the trial after large amounts of investigation and debate, and end up having to do trial by combat instead. This is the option for wizards who put cross-class ranks in diplomacy because they expected better of the man who wrote Planescape: Torment.