What about any combat situation that has nothing to do with a quest, will you engage in optional combat for no reward? Of course not.
If only reward for combat is a little amount of XP, I wouldn't engage in it anyway. Enemies should be obstacles on your way to achieve something. Placing random groups to fight as a means of leveling, because devs are too lazy to think of some more interesting ways to let players get XP, is just poor design in non combat-focused RPG.
Will you try to slip by every single combat situation unless it is a boss with good loot? Of course you will.
Again - if this is stupid filler fight out of nowhere, I would avoid it anyway if I could. But if these enemies look like they can have a good loot, or they are blocking some paths i would like to explore, I would fight them. And I'm talking about mindless monsters of course, because with sentient beings you can find infinite amounts of ways to make combat personal and interesting, so the players won't care about XP.
Will combat feel unfulfilling because the game will force combat on the player in certain situations and not reward him accordingly? Of course it will.
Yes, it will. If the game would force combat on the players and wouldn't reward them. But it will reward them. Rewarding will be just handled differently than in combat-focused RPGs.
... But you seem to know better, D:OS can apparently never be a great game because it rewards the player for combat and in turn doesn't make half of the game completely redundant.
Lol, I never said anything about D:OS. D:OS is great heavily combat-focused game, fighting is the main selling point, while story is less important here. So of course it should have combat XP.
PoE and new Torment, on the other hand, will be more narrative focused games, where combat won't be a goal of playing, but one of many possible approaches, a way to get things done. And in that kind of RPG, lack of combat XP makes sense.
RPG is a very wide genre, there are heavily combat-focused H'n'S-like titles, and also those focused on narrative, closer to adventure games than to combat RPGs. So of course some things which are great in one RPG, can be a very bad idea in other.