A fair point (though technically sudoku can have multiple solutions, they are all equally correct; there is no range of correctness, it's just binary). However, I think something like this is useful as a proof of concept for a GA system, because it's easy to judge its solutions. Many textbook examples of GA I've seen involve evolving a best fit line for a given line, and the simple examples use functions like y = x^2+1 because its easy to judge completion.