That's the whole point. While lots of people are coming to ME3 having played both ME1 and 2, there's always going to be people who play ME3 for the first time without ever having played the earlier games.
James is basically there so he can ask Shepard questions, or Shepard can say things like "Hey James, let me explain this thing to you", in a way that doesn't make sense for most of the other characters (who've either been with Shepard since the first game, or are EDI who is basically a supercomputer who can figure stuff out way faster). And to talk about galactic politics when you're on the mission to recruit Garrus (where they talk about all the races and why some of them don't like the others, to help introduce new players to important aspects of the setting).
He's also there to hero-worship you a bit, to emphasize early on just how much of a big deal you are.
You could argue that he fills a similar role to Kaiden/Ashley (he's basically a male Ashley analogue in terms of perspective and combat skills, and basically falls into the de facto "speaking for the human race" role), but both Ashley and Kaiden will have a lot of baggage with you at that point, and aren't going to react to things the same way James does. He's more likely to call attention to things they'd be more blase about.
Also keep in mind that, even if you ARE someone who's played ME1 and 2, depending on how you've been playing the earlier games, there's a good chance that Garrus, Tali, Kaiden, AND Ashley could all wind up dead in your game. Which would basically leave you with Liara, EDI, and James. And maybe Javik, if you have the DLC. So there's definitely a need for a more grounded character both in terms of combat and dialogue, and James fits that job.
You can't have both Ashley and Kaiden dead.
Also even having him be a newcomer doesn't mean he, somehow, has to be a blank slate meathead. Even something as simple as making him an animal lover with a fondness for dogs, someone who admires Turian culture, and enjoys cooking could have gone a long way towards fleshing his character out and making him seem, well, like more than the token meathead.
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I am the gentle hand who heals, the happy smile who shields, and the foot that will kick your ***! - White Mage