"Along the way, we bought the story that giving someone a hundred dollar bill as a gift ("go buy what you want") is callous, insensitive, a crass shortcut. Buying them a $100 Best Buy card, on the other hand, is thoughtful. Even if they spend $92 and have to waste the rest."
That extra bit... that's the kicker isn't it? You total up what's in your basket and you're three dollars short of the magic number even after you've calculated the taxes and then you look around for an impulse buy to round it out.... batteries? candy bars? socks? the latest National Enquirer? Ooooh, a new ice scraper in a fleecy mitt! And then suddenly, you're $17 over the value of the "gift card" holding bags overflowing with unnecessary crap.
To be fair, if you choose not to spend to the limit of the card, some retailers will actually refund the difference; most will hand you back a card with a balance that may well sit in your wallet until it gradually expires -- a most weaselly trick. (I currently have a mall gift card in my wallet with a $5.14 balance that I continually forget even though I am in that mall probably twice a month. I should probably check the expiry date.)