If the whole thing was a dream, then absolutely everybody could have been a projection. That's why I want to resist that idea--because all of the characters and events would just be products of Cobb's subconscious. If nothing really happened and nobody was real, then the audience can have no connection with the story or the people in it and nothing matters. (I got a bit of that kind of feeling from the excellent interview with Dileep Rao.) If there really was a reality but Cobb stayed in limbo in the very end, then at least the stakes were real throughout the film and the outcome actually mattered. Even if it ends in limbo, there really was a reality he could have returned to, and that reality was witnessed by the audience.
The part of me that likes happy endings would argue that since he sees the faces of his children in the end, that it was real--because the "projection" children in his memory were faceless as they didn't turn to see him before he left. If it was still a dream, he wouldn't have been able to see their faces, right? I'd like to think so.