Ditto - many of the faculty aren't the regular Exeter staff and attendance doesn't give a student an "advantage" during the regular admissions cycle.
But it is a good way to determine if your child will adjust to boarding school. My own D had been pushing us to let her explore the concept - unbeknownst to us she'd been looking at various websites for several years. So we enrolled her in Access Exeter to let her get a flavor of boarding school experience. She took the CSI module and called periodically to say how hard the work was but how much she was loving the independence and chance to test herself. The summer program also allowed her to immerse with students from all over the world and suddenly her social network expanded from the friends at home, to international friends as well. Her roommate was from a different country and they not only melded their cultural differences, but stayed in touch when it was time to apply to regular programs. In fact - for me - that was the best benefit - the ongoing discussion among summer school friends about the pros and cons of different schools they were investigating.
While our personal experience may not be reflective of everyone's I'm pretty much a fan of letting students test drive boarding school this way - with the caveat that the "real" fall and spring academic experiences will be ten times harder. But somehow, her "entry" into BS full time was aided because she'd already learned some of the cultural norms and cues in advance (living in a dorm, communicating with dorm heads and teachers, navigating block schedules, deadlines, different cultural approaches to a problem, etc...)
Good luck. Let us know how it goes.