1. They don't share balls so you can't sabotage them when the other team is on offense. 2. Clearly if they were underinflated on purpose, the purpose would be that they benefit the offense, regardless of the effect you believe it had looking at one half versus the other. 3. The balls would have been deflated after being checked by the officials but before the game started, hence before the Pats had a comfortable lead. 4. "Dink and dunk" passes are easier to drop because there's less time to react and the ball has more of its speed, and the underinflated balls would be easier to catch and get a grip on in cold weather. Tom has already stated (and I'm sure most QBs would agree) that underinflated is easier to throw.