To me, after some bad experiences with online sales, what I get from the IUP is a known amount that I will "get" (I put that in quotes because what they're really doing is forgiving the old loan) when I hand the old phone to a Genius at the Apple store. Could I get more if I worked at it? Yes - but BTDT. I'm paying something for simplicity, speed and certainty, and I'm ok with that.
The thing I don't like about the IUP is being required to pay for AC+, which I would probably never buy for a phone I owned rather than financed...although with the cost of replacement for cracked rear glass panels on the iPhones with inductive charging, it's not as crazy as I thought it was when the back panels were aluminum (or when you could self-service the glass back on a 4).