I'm not saying demanding perfection is a reasonable position to hold and I don't generally expect my politicians to be flawless paragons of incorruptibility. I'm saying that when a politician you support DOES screw up in too big of a way for you to feel comfortable to continue supporting them, the "Well he's still better that Jim Whats-His-Face over there with WRONG LETTER next to his name" line that your fellow party voters will try to feed you is a harmful way of thinking. Straight-ticket voting makes it easier for members of both parties to both engage in corruption and go unpunished when they do, because they know at the end of the day that the people voting for them care more about what side of the fence they're on than what they've actually done.