I noticed that Mako didn't even get any lines in The Original Airbenders and had pretty little appearance in Old Wounds, which does help the argument that he's a superfluous character. His subplot in this season being pretty unnecessary, even with Varrick being so awesome, helps with that a lot as well. The only good thing about his appearance in season three is his moments with his family, although I could kind of laugh at him in a couple of other cases. If he was written out of the show, I don't think much of anything of value would be lost. While I'm not much of a fan of Bolin, I could see him carrying his weight on his own relatively well.