One idea that comes to mind is to use them as a transaction ID to bind the sender to the recipient instead of generating a unique Bitcoin address per sender. If done carefully, this may allow more versatile use of vanity addresses.
Since the address space of the transactions is relatively small (only 5 digits to play with), one idea is to use a random 4 digit suffix that is valid for a short period of time. Use the final / Fifth bit as a checksum that can also be ommitted if the conversion rate BTC to USD exceeds $100 but is below $999
- An item costs 10 BTC, and the buyer clicks "check out"
- The seller creates a new transaction ID (9998) records it, and makes it expire in 1 day.
- The seller calculates the checksum. Since the sum of 9+9+9+8 is 35, an odd number, the checksum is set to "1". (if it had been even the checksum would be "0")
- On checkout the seller pays 10.00019998 BTC to the seller, possibly to a vanity address
- The payment is received by the seller and visible in the transaction history.
- The seller does checksum of the transaction (9998) with the checksum bit (1) and passes
- The result is even so the transaction is valid (less likely a typo was made)
- The seller looks up transaction ID 9998 in the database. If the Tx is invalid, or the date is greater than 24 hours, then a refund is sent to the seller.