While you are correct in that you'll be paying with deflated dollars, inflation doesn't really come into play much over a 24-month loan. If it were a 30-year loan, sure.Read up on Time Value of Money. With a zero percent loan you are paying it back with deflated dollars, I.E. the dollar you pay with in two years isn't worth as much as a dollar today is.
That said, there is an extra cost for Apple Care Plus that you have to take into account. If you were likely to buy that anyway, then TVM applies to that purchase as well. If you weren't, then it is hidden interest.
Me? I have a toddler and another baby due in January.....I'm buying AC+ for both phones I'm buying tonight so the IUP plan works great for me.