I'll be "that BGO."
In the past three years, I've had at least one candidate each year (sometimes more than one) with an LOA who ended up with no nom. And a couple with only a Pres nom in a state/district where MOCs will give MOC noms to those who already have Pres noms.
In one case, USNA gave the candidate a Supe's nom (the candidate turned down the appt). In the other cases, the candidates received turndowns. I can't say whether my experience is unusual or the norm -- but it is reality.
The above said,
Memphis is correct -- the overwhelming majority of LOA candidates will ultimately receive appointments. So, yes, it's a big deal to get one. And for those eligible for Pres noms, it's a HUGE deal as Pres noms are automatic for those who qualify. So all you need is to clear medical and pass your CFA.
The downside is that one cannot do anything more to get an LOA than he/she can to get an appt, which is to put forth the strongest record possible.