I think Divinity 2 is an RPG and sure does benefit from NOT having respawns. Why?
1. There is no fast travel or teleportation in this game, neither can you fly as dragon everywehere. Because of this, it would be very annoying if you had to fight the same mobs over and over again. Imagine yourself becoming stronger and the mobs staying the same, you'd be butchering them, which is no fun. But if the mobs level-up just like the player, you will have the feeling your character is not progressing at all.
2. The enemies are pretty hard to defeat, especially in the beginning and for inexperienced players. I am happy they stay dead when I defeat them. It would be horrible if they were to respawn again somewhere I had already passed. This also gives me the feeling I am actually doing something in a REAL world. Respawning mobs is just so... lame... and MMO or multiplayer RPG-ish.
3. By NOT having respawning mobs, the player is more stimulated to do quests and solve them for the extra XP. It would be silly if you could reach the maximum level or max out your favorite skill by killing a mob over and over again. Now you have to explore the world, take risks and fight new enemies. That's so much more fun than just holding down the attack button, right?
Because Divinity 2 is like this and not like, say Sacred 2, it's a different type of game. A different type of RPG. Considering games ranging from Fallout 3, World of Warcraft, Sacred 2 and Borderlands are called RPG, I wouldn't see why Divinity 2 isn't one!