Character Dimensions... human dimensions?

Today in class we discussed the difference between a 2-D character and a 3-D character, specifically using examples from The Adventures of Huckleberry Finn which we are currently reading. To sum up what we came up with in class, we decided that 2-D characters are unchanging, stereotypes, shallow, and linear. While 3-D characters were well-rounded, adaptable, and often have a back-story. Obviously there are other qualities that could define these characters, but this was our outline. It got me wondering, though...

Do these "character dimensions" say anything about real people?
In many cases, I think that characters from books are based off of real people, or they are a combination of people that the author may know. Therefore, can one assume that the "dimension" of the character could be applied to the human model? I know you can call someone shallow or stubborn (unchanging), but do those characteristics automatically place them in a broader category... I've never heard of a person referred to as 2-dimensional that I can recall.

Also... Are the main characters usually the main characters?
As a class we decided, rather unanimously, that Huck and Jim were the only 3-dimensional characters in the book thus far. All the other, more minor characters, seemed to fit much better with our description of 2-D. I wonder if this is always the case or if this is often the cases in books. Perhaps it makes the books more interesting or more relatable to the reader if the primary character in a book is something other than a shallow, undeveloped personality.

I am rather undecided as to what I think on both issues, but what do you think?