There are 2 reasons to attend SLE along with NASS. The first is that you do have some interest in USMA. Maybe not as great now as USNA but still some. Seeing WP up close and personal might change your mind or at least make you interested enough to apply to WP.
Second, you want an additional life experience, can afford to attend, and don't have anything "better" to do that week. I'm not joking about this -- you'll learn something and it will be an interesting experience.
The major downside of attending if you have zero interest is that you're probably taking a slot away from someone who really wants to attend USMA. Also, if cost is an issue, why waste your parents' money on something that doesn't interest you.
As an aside, it's my understanding that all SAs "count" attendance at any SA summer program equally and don't give double credit. IOW, if you attend NASS, that still "counts" for USMA purposes as much as SLE. Ditto for USNA and SLE. In addition, at least for USNA, you get "credit" for one summer program; attending 2 or 3 doesn't benefit you more from an admissions standpoint.