An LOA should be the farthest thing from your mind at this point in the process. If you search "LOA" on this forum, you will find many threads and comments regarding them.
However, the "recipe" that resulted in an LOA last year or the year before may not be on the menu this year! There is NO WAY you can know what the review board wants/needs and is willing to extend an LOA for this year. Maybe a highly qualified candidate (think awesome test scores, GPA, sports, leadership roles) is from an "under represented" district. Or maybe that same highly qualified candidate is being highly recruited by Harvard or West Point or another school. Maybe this highly qualifed candidate is a heck of a linebacker and Navy has a need for a linebacker in the Class of 2014. In short, there is absolutely no way any candidate can assess their chances of receiving an LOA.
It is reported that 100 or so LOA's are extended each year. 1500 or so appointments are offered. Appointment is the goal, the only goal.
So, do not give it another thought. Get your application in early and in the best shape possible. Get after the applications for nominations from your MOC's. Fill out the application for VP nomination.
On your question about how MOC's treat the LOA.......I think it varies. Some want their nom to go to someone with an LOA while others figure the Academy will find another nom source for the LOA recipient, so choose other candidates. Some, most from what I have learned, submit an unranked slate including those with LOA's and let the Academy's choose who to extend offers of appointment to........
As difficult as it sounds, do not obsess over the LOA. Tell yourself over and over that you will not know anything from USNA until March 2010. Then, if you find out earlier, it is a bonus! If you need to beef up your CFA, get to training so you can update the CFA scores. Ask for a Candidate Visit Weekend once the academic year starts. Keep in touch, though do not "bug" your BGO.