SLE is a great opportunity to see WP if you haven't been there, and get sort of a feel for what life is like as a cadet. SLE is really designed as a recruiting tool, to expose the service academies to people who might not be familiar with them, so if you live in an area that is well-represented by candidates, or if they know you have another academy connection you might not get a spot (or they might just run out of spots, since budgets are tight, and many qualified people apply).
DS didn't get accepted to SLE last summer, no real explanation other than he was qualified but they didn't have enough spaces for everyone. Perhaps it was because he has an older brother already at a Service Academy, who knows? While we were a bit surprised, not going to SLE opened up the opportunity for him to attend Boys State, which was a wonderful experience. He also attended the USMC Leadership and Character Development Academy for a week at Quantico. He was offered an appointment to WP (2018) in January, so not attending SLE didn't seem to negatively impact his Whole Candidate Score.
Bottom line, the summer offerings at the Service Academies are great, but not the only leadership opportunities available - search around and you'll find some great alternatives!