There is $8500 worth of stuff that is issued to plebes on I-Day. The Academy doesn't want to make plebes front $8500 on I-Day, so they use the ACE loan. If you don't like it, you can go ahead and give $8500 to the financial office during the ac year; the Academy doesn't care. It's zero interest though, so there is nothing to be gained from doing that.
Pay is held month to month mostly for those few months out of the year where the $700 - $800 of remaining midshipman pay is not enough to pay for the expenses for that month (engineering majors during book issue, plebes during SDB jacket, O-Coat, and Reefer issue). To ensure midshipmen are still paid something month to month, any deficits are rolled over to the next month and paid off with excess held pay. Held pay is released in January and August.
We're the only academy that does this. Other academy cadets can get a lot more money per month, but other academy cadets will also have months where they are not paid because there is nothing left after deductions (my understanding from a conversation with an USAFA cadet, anyway). So no matter how much I buy in the bookstore, uniform store, or Mid Store, I can count on getting that $100 as a plebe and the debt is just rolled over and paid off with a future month's pay.
I used to think the held pay system was pretty dumb, but there's benefits to it.