The exams can be effective as both a full-length "mock" experience to get used to taking an entire exam, and as subject or passage-based practice. I think you need a lot of the former (full-lengths) to get uber-comfortable before exam day. You can also use exams like a section bank/passage bank. I cannot speak to the NS exams, but I know the Altius exams will let you set the delivery method at the start to show you only one section, or only passages/questions from one discipline or topic. I'd recommend taking at least 5 of them as full-lengths, then you'd have another 200 passages to use for regular passage or topic-based practice.