I don't think any of them are very developed, but I do think Arthur, then Ariadne are the flattest - they seem to have virtually no existance outside the plot. (Yusuf has his basement of dreamers, Eames has his Mombasa hijinks to indicate a larger world they partake in.)

How is being Cobb's point man for years a less valid background than being a chemist or living in Mombasa? Arthur must have a hell of a network to be a good point man and he learned those martial arts skills somewhere. And Ariadne studied to be an architect - they all had lives outside of Inception. You may think their backgrounds are less interesting, but that's another matter.
Characterization and background may be connected, but they're not one and the same.