I find myself using the expression editor to define my Enabled State condition infinitely more than the default Boolean Variable functionality.
Specifically i use !ISNULL(@variable.Input_Field) or !ISNULL(@table record.Table.Id) nearly constantly to make sure a field is filed, or a record was properly loaded.
It would be nice if such a simple use case were a default selectable option. Though it might require a subfield i.e. Enabled State > Not Null > Variable/Table Record