I must be a weirdo but I love it when artists leave the bluelines on their work. It's neat to see the defined form of the finished piece and the blue (sometimes red or another color) linework underneath showing the framework that went into creating it.
I guess that is one of the reasons I collect sketchbooks from my favorite artists. I get a kick out of seeing the rough layouts and the problem solving that my peers and idols use to figure things out on the blank page. I personally feel it gives the work a gritty, earthy vibe-something about the building up of the lines and the narrowing down of the choices.
Or maybe I just like the color blue.