NASS is first and foremost a marketing/recruitment program. The figures you mention above are misleading if someone were to read that the acceptance rate is 50% of applicants who attended NASS, when the accepance rate for others is roughly 10%. While there may be a correlation between attending NASS and completing the Application, and acceptance at NASS does have some prescreening so the NASS candidate is at least competitive, so attending NASS may be a positive indicatior, the converse (ie. not attending NASS is a negative) is not true. USNA1985 makes some good points on how NASS can help or hurt if all other points are equal, but failure to apply for or attend NASS, are not held against you.