Luigi - yes you are totally right - LOAs serve an important purpose in the admissions process. If it sounded like I was saying they are not then I mis-stated it. What I was trying to do is make sure that candidates understand that it is great to get an LOA but it is not the end of the world if they don't get one.
Yes, MOCs are given notice about LOAs one to two weeks before the SA tells the candidate about the LOA. This is a way for the SA to allow the MOC to notify the candidate directly. Some MOCs take advantage of that early notice by calling the candidate to tell them the good news directly and some don't.
While it is rare, it is possible for an LOA not to get a nom. In most cases, the MOCs do nominate the LOAs. However, in highly competitive areas, I have seen a MOC with more than 10 LOAs - there are only 10 spots on a slate so anyone after the 10th spot doesn't get that nom. Yes, the SA has the authority to give that LOA a nom - and usually do. I know of situations where a candidate with an LOA did not get a nom and the SA chose not to give a supe nom as well ... if the candidate does not do their part (apply for all noms for which you are eligible) the SA may see that as lack of interest/focus or whatever.
I think it is safe to say that if an LOA recipient does everything they are supposed to do (pass DoDMRB, finish HS without a bunch of Fs, doesn't get into trouble with the law, AND TRY to get a nom) they'll get the appointment. Let me give a few examples that I know of where an LOA did not get an appointment. Off the top of my head I can think of several instances where an LOA was later voided.
1) Problems with the law (senior prank gone bad)
2) Failure to secure a nom (did not finish nom application)
3) Failure to secure a nom (nom board made conscious choice to refuse the nom & so state to the ALO and SA directly for attitude)
4) Failure to secure a nom (more than 10 LOAs and the student did not apply to all sources for which he was eligible).
The third example I heard about but do not have first hand knowledge of - the others I have personal knowledge of. There is one more example too that I saw first hand as well. Candidate had told MOC first choice was USxA and 2nd choice was another SA. MOC gave nom to both choice 1&2 but not choice 3. Candidate changed mind and had an LOA to choice 3. Candidate had already interviewed and told MOC panel 1st and 2nd SA choice and they gave two noms. Around Feb/Mar candidate asks for nom to be switched to 3rd choice, for which candidate also had an LOA. MOC refused to change the nom (slates are submitted by Jan 31 so this is after the slate is submitted). In his interview the panel very specifically asked the student to rank the SAs to be 100% certain what he wanted. The first choice was adamantly the same, #2 was a distant second and #3 was not even on the radar. For whatever reason, the student changed his mind to #3. But, without the nom, his only available option was to wait a year and try again next year or go to his now 2nd choice. The #3 SA did not give this LOA recipient a nom. By the way, that was a couple of years ago and the young man is thankful for how it worked out as he is, "where I belong".
Obviously this is an extreme example. The reality is that LOAs are a great thing and serve an important purpose for the SAs. However, for the candidates that don't get them - it is not the end of the world.
As to the other poster asking why not focus on an LOA - well, how about instead of focusing on the LOA you focus on the best application possible? Kids that focus on the LOA risk disappointment. A student in my D's HS whom I know well did not get an LOA when he was applying and every single day that he did not get an LOA in the mail was as if he was being rejected. He became depressed and it affected his senior year. He was a very strong candidate for a SA (ranked top 2% of the class, SATs over 1450 range, 4 yr varsity sports and team captain, strong leadership etc) but in the end not only did he not get the LOA for which he thought he was qualified, neither did he get an appointment. He spent his senior year stalking the mailman and not planning for "Plan B". He ended up at the local JC. Two years later he has his AA degree and did not go on to get his BA - instead opting to enter the labor market and begin his working life.
Maybe it is because I know this young man and I watched him agonize over not getting an LOA and then wait for spring and not get an appointment. I see his parents every few weeks and I know he/they still thinks about it. They don't ask about my D at USAFA nor do I volunteer anything about the opportunities she's had. Don't let that be you - enjoy your senior year. Make your file as strong as you can, and update it as often as necessary to ensure it reflects your achievements but don't obsess over it. And, if you do nothing else - apply to a Plan B college!