To be imperfect or not to be imperfect...that is the question
"And yet, by heaven, I think my love as rare
As any she belied with false compare" (Shakespeare 13-14).
Good thing Shakespeare threw those last two lines in there, because if he would have read this aloud to his mistress, she probably would have slapped him after the first stanza. :-P
Seriously though, Shakespeare finds beauty in imperfection. He is making his mistress real to us in this poem. How many odes or sonnets have you read that make the woman the narrator is in love with out to be a goddess? Not that there is anything wrong with that, but the fact that this sonnet does not follow that pattern is what makes it unique. He is pointing out her flaws to show us that she is a real person. That makes the ending (where he confesses his love for her) much more significant. He is saying despite all her imperfections, he still loves her. If he described her as a glowing goddess with beauty like no other and then at the end said that he loved her, we'd be like, "Yeah, well, who wouldn't? Every man in town would be after her." When he writes in this way, he makes his mistress much more important to us (as readers).