Always returns true. The reason the method is here is that both joint.dia.Element and joint.dia.Link inherit from joint.dia.Cell. This method is useful if you don't know what the cell is. Calling cell.isLink() is equivalent to cell instanceof joint.dia.Link. Example:

var cell = graph.getCell(myId)
if (cell.isLink()) {
    // Do something if the cell is a link.