JavaDoc reads: "The cell that is returned is a {@link Cell#CELL_TYPE_BLANK}. The type can be changed either through calling setCellValue or setCellType." But that is probably incorrect - why should I specify type if type for returned Cell is Cell#CELL_TYPE_BLANK? In all HSSFRow, SXSSFRow and XSSFRow implementations the implementation is correct (it sets the type), so please fix the JavaDoc.
Good spot, fixed in r1479639.