A little understanding:
Obviously, the academy would prefer that they could have complete say so over the 1100-1200 appointments given out each year. They'd probably prefer that there weren't even nominations; similar to how the coast guard selects. At the very least, that all nominations were competitive and there were no principal or ranked nominations. But all states are made up of tax payers and each state therefor has the right to have some of their taxpayers represented at the academy. Plus, by guaranteeing each district/state an appointment per slate, the academy also gets geographical diversity. Of course the academy could ensure this without principal or ranked congressional nomination slates.
But as it is, approximately half of the appointments come from congressional slates and the other half of the appointments the academy gets complete say so either from military related nominations like presidential, rotc, etc. as well as the "National Pool". So as it stands, it's a pretty fair distribution in my opinion. Even if all 535 representatives and senators used the Principal method and gave the academy no say so on 535 qualified appointments, the academy would still have complete control over the remaining 600-700 appointments.
Having said all that, that is why the academy would love for anyone that they gave an appointment, using a presidential nomination, to move forward and STILL apply and RECEIVE a congressional nomination. This means the academy doesn't have to use the presidential nomination for YOU and your appointment. They can give it to someone else in a more competitive state that for whatever reason, their representative or senator didn't think they were good enough to receive a nomination.
But as already mentioned, once you receive an appointment and accept it, it can't be taken away from you unless you screw up. The academy can manipulate and change which nomination it is slated against, but they can't take it away from you.
Scenario: You have a presidential nomination. Your application walks on water. In October, the academy decides to offer you an appointment because they really want you and they know you would get accepted to just about any college you applied to. So; using your presidential nomination, they offer you an appointment and you accept it. Now, to use a simple state that only has 1 representative; like Wyoming or some others, there are 2 senators and 1 representative slate being presented. So you state has a maximum of 3 possible appointments using their nomination slates. If you already accepted an appointment with the presidential nomination, and you DON'T take a congressional/senator nomination, then your state still has 3 individuals who should receive an appointment, plus you (with the presidential), equals 4 total. If you also take a congressional nomination, then the academy can MOVE your allocation FROM presidential to your congressional nomination and give your presidential slated appointment to someone in say Georgia, Florida, Texas, etc. So your state only has the 3 appointments instead of 4.
So, the decision is totally up to you. Do you want your state and citizens to get as many appointments as possible, or do you want the academy to give out more of the appointments? That's what this all comes down to.
Mind you, this only applies to an individual who HAS RECEIVED AND ACCEPTED AN APPOINTMENT. This has nothing to do with having a presidential nomination but you haven't received an appointment yet. You definitely want to apply for as many nominations as possible in case you aren't one of the luck 100 presidentials' or one of the lucky congressional/senator slated ones. This scenario is ONLY applicable to individuals who RECEIVE AND APPOINTMENT PRIOR to congressmen and senators interviewing and giving out their nominations..
This EXACT SCENARIO happened in my state when my son applied and the next year. My son had a presidential; was offered an appointment at the end of october, accepted it; informed the congressmen and senators he didn't NEED a nomination and please use it on someone else; and out state had 4 appointments. 3 congressional/senator and my son's presidential. The very next year, the same thing happened with another applicants, accept he still attended the congressional nomination interviews (Even though he already accepted the appointment using the presidential in October). He received a congressional nomination. He still kept his appointment, but the academy took the charge away from his presidential and used the congressional nomination and gave the presidential slated appointment to someone in a different state. So our state only had 3 appointments that year instead of 4. If he didn't have the congressional nomination, the academy wouldn't have been able to move his appointment to a congressional slate; they would have had to KEEP it using the presidential; and the state would still have had 3 other appointments.
So that's how the game is played when it comes to using slates. Some don't think it's a problem. That the academy should be able to do whatever it wants. But it's also common for a congressional slated appointment for the individual to turn it down in May-June. The academy "SHOULD" be required to go back to that congressional slate and pick another qualified from it if someone on the list turned down the appointment. But if they can move someone with a presidential TO that list, then the academy can use the presidential somewhere else. So; now you know.
But as stated by others, once you ACCEPT AN APPOINTMENT, it doesn't matter if the academy moves your slated spot to a different slate, you still CAN'T LOSE your appointment. The only way that can happen is if YOU SCREW UP before entering basic training. And again; this only applies to individual who receive an appointment PRIOR TO MOC NOMINATIONS. If you already have a MOC nomination before getting an appointment; there's a 90% chance you will receive the appointment with the MOC nomination; NOT the presidential.