You'd accept IOUs for three reasons:

1. The payer may not have XRP, since those are in finite supply and take effort to acquire.

2. XRP, like BTC, isn't be a great unit of account, since its value hasn't stabilized yet.  Unless you want to constantly be keeping an eye on the current XRP price and doing conversions back to more stable units, like dollars, or better yet, something like Terras, in your head.

3. You are unhappy that a small group controls the majority of XRP and you'd rather not use it for payments.