Why would you take BG2 as the epitome of AD&D implenmentation? There are no skills, the world is unchanging and lifeless, and there are very few ways of interacting with the world other than through combat. Even the robust spell system is gutted of almost all the classic spells that allow you to interact with the world.
By AD&D I mean 2nd edition, where the p&p way to handle skills was either to roll the skill's corresponding attribute as needed (i.e. CON for a swimming check) or to just say that your character has whatever skills and knowledges that you have. By epitome I mean that compared to the Gold Box games, it created a true rpg world within that rule system. I'm not comparing it to 3E rules games like NWN/ NWN2/ ToEE, which have explicit p&p skill rules.
I don't think you need classes or skills, just a well simulated world and methods of interacting with it by some means, whether that be attributes, skills, profession, alignment, prior knowledge found in game etc. It doesn't matter the method, just so long as the interaction is there and we have the ability to approach the game through some means other than combat. Look at Underworld and the Ultimas, interactivity was off the chart, and it used different approaches in almost every game.
I still remember using a pole the first time in Underworld to press distant objects I couldn't reach, finally a game implementing the pole 10' item that was a staple of AD&D inventory, now that was an epitome of interactivity.
I don't think you get my point. My whole point is that you do need classes, and when you have a system where skills replace character class, that system is fundamentally flawed. Your character in a RPG isn't a generic avatar like in a FPS. It's a role that through which to experience a story based on literature and history.
Why does D&D have a priest class? Because Western literature has Gods, and people who interact with the divine, whether it's Moshe, Jesus, Muhammed, Daniel, David, John the Baptist, Noah, Cassandra, Archbishop Turpin, etc. Literature defines that role. Same with Rogues- Robin Hood, Odysseus, Jean Valjean, Edmund Dantes, etc...
When FASA created Shadowrun, they certainly didn't need to make both Mages and Shamans, but they did since they understood that these class roles are central to RPGs.
That's really always been the flaw with GURPS. GURPS didn't understand that classes are central to RPGs. So while it's true that you don't need to have classes to have a good open-world game, classless rpgs miss the entire point of rpgs. On a certain level, New Vegas might as well be GTA V or Assassin's Creed. And actually both GTA and Assassin's Creed provide you with a character to play as just like Twitcher. Modern RPG design philoophy which allow you to customize your character as you want and have no class system to restrict and structure your character progression (like Wasteland 2) miss the entire point of RPGs. Open world interaction is nice but that's not the essence of RPGs. GTA V and Assassin's Creed do open world interaction just fine. Nor is customization the point of RPGs. The point of RPGs is experiencing a game from a role, i.e. class.