I agree with a lot of what you say. Adding to the difficulty of picking one SA on the go in is that a lot of the programs to "get to know" the SA (i.e. overnight visits, diversity visits, CVWs,etc) are not available to the child unless he/she applies. So, its hard to narrow it to one at that stage of the game because its hard to have all of the necessary information. I am pretty sure you can't get an inside look at USMA (i.e. overnight) unless you have an offer.
What if the 18 year old is certain they want X career but when the get to the academy that feeds X career they hate the place and ca't complete the necessary 4 years to get to X career.
I agree that the nomination facet of the process is where it gets messed up. A kid goes through all of the trouble to apply and visit and interview and complete all of the applications for each SA, only to have the MOCs force them to rank or list only 1 and then coordinate noms to make sure they only get the one opportunity.