We get asked all the time if Japanese erasers can actually erase. This question usually comes from the purchasing parent of the child Japanese eraser collector. Almost always the child will remark something like, "You don't erase with them!".
Considering so few of us use pencils nowadays, the paintbrush eraser probably will not see any more use than the collectable Japanese eraser. Practicality has never been a criteria for the stuff on sausalitoferry.com. anyway.