Bingowings said:

Artoo and Threepio are fully rounded characters (arguably more so than Anakin and Obi-Wan) and yet they can be sold and ordered around and generally abused even by the heroes.

R2 and 3PO are not fighting droids. Their concept and role is completely different than that of the combat droids and clone troopers. R2 and 3PO will always have more character and appeal than an entire combat droid army and clone army combined (no matter how well the idea was/would be executed).