Incomplete arguments

Ever have one of these arguments?

Person 1: "We should not do X because despite A and B, C prohibits it"
Person 2: "Yeah, but we'd get A and B!"
Person 1: "Uh dude, C?"
Person 2: "A and B are great!"

Yeah ... jackass, we agree there, but you didn't address C. A specific example comes from a message board on an Arizona Cardinals's news site, in a story explaining that despite the [intentionally unspecified] compensation they'd get for Larry Fitzgerald, due to esoteric regulations the Cardinals would still have to eat $15 million of cap money, thus making moving him a non-starter. One intrepid reader comments "Trading Larry works for all parties! The Cards are clearly in rebuilding mode and get a ton of value for him, and the other team gets a great player!".

Mr. Reader added a new piece of information (the Cards are in rebuilding mode) that has no relevance to the original disputed statement and thus adds no value. He then agrees with 2 of 3 statements made by the original author, but disagrees by simply omitting the author's 3rd point. Try this one on for size:

Person 1: "There's freedom on the other side of that concrete wall, but if we make a run for it we'll definitely get mowed down by about 100 bullets"
Person 2: "Your hair is red. You should do it. There's freedom over there."

If you want to invalidate someone's statement, address the points they've made. You can bring in new information if you tie it to what you're saying. If you act like the dude above, you just look like you haven't been paying attention at all, or like you're comparing a banana to the speed of a cheetah. Huh? Exactly.