The key word there is "their". If you want to be an apprentice, then a non-tenured professor could be fine. You're working on their, narrowly conceived, research. Of course, there are exceptions but an applicant to grad schools isn't usually well-prepared to distinguish. So, given your last point - which I agree with 100% - I think it's better for students to just go with more established folks (assuming good reviews from their current and former students). Let the non-tenured faculty hire assistants - they're cheaper and their independence is less critical to their long-term development.
