To follow up on USNA69's EXCELLENT post, getting a Pres nom is easy -- receiving an appointment w/only a Pres nom is difficult. The reason is that the nom itself is essentially non-competitive. Thus, almost everyone who qualifies for one gets one. The problem is that a lot more than 100 people qualify. Thus, if that's your only nom, you compete against the potentially hundreds of other folks for those coveted 100 appointment slots.
I have had a VERY highly qualified candidate (1400+ SATs, AP courses, varsity sports, etc.) not get an appointment. When I asked USNA why, one of the reasons I was given was that the candidate only had a Pres nom (other was needing an eye waiver; the combo was apparently not good).
A couple of other points. USNA wants to see that you're willing to do the work for the other noms. Generally, MOCs require more data, interviews, etc. than the Pres application. Not applying for the MOC noms could be viewed as trying to get out of the work, that's not a good sign. How does USNA know? -- BGOs generally ask candidates. I do.
Second, per my first comment above, MOC noms are competitive, and USNA likes to see that you can get a nom competing against your peers. I realize that, in some geographic locales, there is less competition than in others. But for candidates in competitive geographic regions, it's a help.
Finally, as USNA69 stated, the more noms you have, the more flexibility USNA has in "slotting" you for an appointment.
All the above said, there are folks who get into USNA on only a Pres nom. However, I would not start out with that as my premise. It has the potential to end in disaster.