In the original Dead Space, Isaac Clarke was nothing more than an engineer. He came along with his pals to see what was going on with the USG Ishimura and to visit with his girlfriend, and he was caught up in the red marker incident at Aegis VII. Though he eventually became the hero of that incident, shutting it down pretty much single-handedly, most of the game he was just running around the ship trying to fix things so he and his crew mates could leave. It was left to him to wander around and do said fixing because, again he was an engineer, and doing those things was his job, and all the other engineers were dead. He fought a lot of necromorphs, yes, but he only did that because they were in his way. His objective was not to fight them.
In Dead Space 2, Isaac was once again caught up in a marker incident, and it was also not his choice to be stuck in that situation. During the bulk of the game he is merely trying to escape the Sprawl so he could survive. He was still not trying to be a hero. He wanted to get away, because he was not trained to fight. He was good with a cutter, but he was not a soldier, and he wasn't playing the role of a soldier.
Isaac in the first two games is akin to Gordon Freeman in the original Half-Life. Gordon was just a scientist stuck at the bottom of a secret research facility when an experiment went wrong and monsters popped up everywhere. The bulk of that game is about Gordon just trying to discover a way out and beasts were tearing up the facility andercenaries were trying to kill all the people. Eventually, though, he goes on the offensive, but it's only because by the end there are like three people left and he's the only one with a Hazardous Environment Suit.
After the first chapter of Dead Space 3, Isaac is a soldier, though. He is recruited to find the source of the marker signals on an ice planet where his ex-girlfriend Ellie is. He has a posse with him, but Isaac, as was the case in the previous game, goes it alone to complete all the objectives in the game. When Norton, the head soldier, needs somebody to scout a new area, he sends Isaac, even though Norton is a trained soldier and Isaac is not. Sure, there are plenty of contrivances that separate Isaac from the group, but even when he is working with them he's the errand boy, and not just on engineering tasks.
It's a remarkably wrong-headed scenario to write for a character like Isaac, but the blow is softened by one thing: co-op. If you're playing with a partner, that person will be in control of Carver, who is a soldier. That Carver goes along with Isaac on this adventure makes sense. Still, I would imagine that a lot of if not most players went through this campaign solo, and so a solo campaign is an equally valid interpretation of the events of the game. I'm not debating canon.
Even with a co-op partner, it doesn't make much sense that Isaac is on the A-team. He really ought to hang back with the doctors while Norton and Carver venture out to do stuff. He is, again, just an engineer.
There is a reason that Isaac must go on the mission, however, and it's because of what the marker from the first Dead Space imprinted in his head. Isaac is special, and we know he must have a role to play in the ultimate victory because of that. But, AGAIN, that he is so special is another reason not to make him wander around alone or with a single partner to do everything. Without Isaac, the group if probably doomed to fail its mission, and yet he's the one who gets the most risky assignments in the game. He is the one who is put in harm's way most often.
In Half-Life 2, Gordon switches from scared scientist to soldier as well, but the transition is smoother. While we know Gordon is an important figure because of the way all the characters, including the G-Man, talk about him, his importance is more as a symbol, and staying with his perspective is important to the storytelling. Isaac, on the other hand, has tangible importance, and his perspective is irrelevant in Dead Space 3 since Visceral decided to disregard that whole thing about how he was going insane.
In short, we didn't need to play as Isaac in Dead Space 3, and it doesn't make sense that we do given the tasks we are given. The scenarios written for the game needed to be reworked in order to insert him logically into these events. The significance of what is in Isaac's head has become a burden on the franchise, and I wish, once again, that he had died on the Sprawl instead of being rescued after the credits rolled. Alas: cest la vie.
