Similar to the Select tool's Unknown Field Checkbox, I figured it would be useful for the Data Cleansing tool to have this functionality as well in order to avoid a scenario where after a cross-tab you have a new numeric field, one of which has a Null value, so you can't total up multiple fields because the Null value will prevent the addition from happening. If the Unknown Field box were checked off in the Data Cleansing tool then this problem would be avoided.