Replace option in workflow Assign Data activity

In a workflow assign data activity, ii appears the the 'Replace' option is selected by default when assigning any type of data to any other type except for a Keyword field of a document.

Is this by design? Is there never an option to leave a field with the value that it already contains, from a previous action or when the document is stored?

I have some instances where if an index field of a document already contains a value I want to leave it as is, but replace it if it's empty. Is my only choice to test the value of the field first and skip the assignment if it is not empty?

Thanks,

Use an IF statement in an arithmetic expression like this

IF(DW_STRING = NOTHING,"New String",DWSTRING)

Which is essentially "If String is nothing, then use the value "New String", else use String (which still has it's value)

The replace option is always there for data assignments that can only hold a single value.