What makes a product owner useful to his company and team is attending every one of these user tests.

使得产品主管对于公司和团队有用的就是参与所有用户测试。

To me it is absolutely inconceivable and inexcusable for the product owner to not be at every one of these tests.

我认为，用户主管不参与到每一个用户测试是难以置信的，也是不可饶恕的。