I play multiplayer against myself or my friends. Mostly myself. And the Great Wall is excellent at the right time (as is the "useless" Angkor Wat, but that's a different story).
The Great Wall makes it extremely difficult for an enemy to invade your territory, and if that land is open terrain, your archers can kite all day. And if it is hilly, then you have plenty of time to harass before the siege hits your cities (because the siege weps need to be adjacent due to hills).
If you are playing a turtling game, there are certainly other wonders that you need. And you still need units to defend yourself. In multiplayer though, I don't think anybody cares about how much you win by, only that you win. So if you need the Great Library, you don't need to rush it and use your super engineer combo to hurtle down the tech tree, grabbing the Hagia Sophia and using that for the Porcelain Tower. You don't need to get there as fast as possible, you just need to get there before your opponents. That gives you time to build the Great Wall, which thereafter provides enough safety that you can go back to your tech-crushing strategy (or whatever peaceful turtle you do).
Human players are competent enough to use terrain, but the Great Wall makes it impossible for them to fight. And even if they could conquer you, it would be easier to take 3 other civs and all of their stuff. And the human knows it. Once you turtle for a bit, then you might be worth it. But by then, hopefully you're unstoppable!