The main problem, as I see it, is that players have cartoon eyes. Correct me if I'm wrong, but while they have different hair and skin colors, the simpsons caracters all have black eyes. I imagine it would look strange if the black dot was replaced by a blue/brown/green dot. A lot of people don't pay attention to eye color anyway. I cannot tell you what color are the eyes of most people I see everyday.
Since there is no distinction between pupil and iris in BB, I can see why this would be more difficult to implement than you may think.
I'm all for it though.
"Air is beautiful, yet you cannot see it. It's soft, yet you cannot touch it. Air is a little like my brain." - Jean-Claude Van Damme