Right on the money, SB! The "A" in "APR" stands for annual. Reg. Z's rules for measuring time require you to divide a year into time periods corresponding with the frequency of cash flows (advances and payments)--up to one year each.
From the income perspective, your bank is losing money unless this "single payment loan" has interest payments along the way. In an interest-only deal, the APR is your yield. If a properly calculated APR comes out lower than the IR, you should require interest payments in order to maintain your yield.
From the credit administration perspective, you also improve the quality of the asset when you can see that the borrower is able to make payments in a timely manner. Waiting for 5 years to see whether the borrower can and will pay you is an unnecessary risk.
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...gone fishing.