1) I see no capability/logical gains, just aesthetic gains.
2) If we can think of ago(), why not a future equivalent? This creates an asymmetry in the patch (ex: for checking future expiration dates, etc..). Using ago('-1') for such cases looks funny, as Andreas already mentioned in a previous comment.
3) It's not that users would be stuck if it's not there, like already mentioned alternative 'current_timestamp - INTERVAL '1' DAY' by Laurenz or say 'current_timestamp + INTERVAL '1' DAY' for future cases.
