Things change all of the time, but at least a few years ago, NASS squad leaders did fill out evaluations. That said, this is primarily for the benefit of the SLs (learning to write evals, which you do a lot as junior officer). In terms of the attendees, they CAN help a tiny bit if the SL says the person was motivated, a good team member, tried hard, was reasonably fit, etc. They could potentially hurt if the person was the opposite (whiner, non-participant, not good team member, totally out of shape, etc.).
USNA recognizes that the people completing these evaluations were plebes a few days earlier. Thus, they don't carry much weight -- they definitely will not make or break an application.
A decent number of NASS attendees receive appointments. But it's not a "stepping stone" or even a block to check for an appointment. As with CVW, it's an opportunity for candidates (or would-be candidates) to learn about USNA. Those who go to the trouble of applying for NASS and attending NASS are generally highly motivated to complete their application, etc. So there is a correlation. But some folks who attend NASS hate it and never apply and some apply but aren't sufficiently competitive.
So why does USNA offer less competitive folks a chance at NASS? As we constantly explain, it's a RECRUITING TOOL. USNA hopes that folks will go back to their communities, schools, churches, Scout troops, teams, etc. and talk up USNA. Even if the attendee isn't super-competitive, maybe some of the folks s/he talks to will be.