Point is that when there are multiple duplicate values in index, find it HARD to delete specific entry matching a particular row. But most commercial database B-trees don't do that. But nobody uses it. Seems to argue against indexes except for table with no updates. But in fact most updates of tables occur to aggregate fields.
Credit card rows.
But not by balance. Might want all customers who have attempted to overdraw their balance but this could use an indexed flag. Anyway, most common change to record is balance. Probably more efficient. DB2 has nice compression for multiple duplicate keyvalues in an index.