Single player game is not interesting, but here you can be taught, how to play MPO+. In MPO+ single player game exists to upgrade your characters**, to get some weapons and items, to play some type of MGS on PSP and to get secret characters (Naked Snake*, Old Snake, Raiden and Roy Campbell, by the way the secret characters can bring rare (maybe unique) weapons in the game (Old Snake – M4 Sompod, Raiden – Mk23, Roy Campbell – some sort of grenade) useful).
You can feel all advantages of MPO only if you're playing in multiplayer (adhoc, or infrastructure mode) you can enjoy: your characters with weapons and items (yes, in the single player game you'll have to seek for them), battle with your friend(s) and chat (by the way you can edit some massages).
*If you played MGS portable ops and still have savedata(better if it's data of a cleared game) then, when you start the new game of MPO+ you'll also be given a choice: download ALL your characters from MGS portable ops or not. In that way you can get Naked Snake, Null, Python and other ones you can get with cheats.
**Some characters are VERY hard to get, so you can enter cheats to get some rare soldiers(like "multicolored" genomes, US state soldiers and others).
A great game, I strongly recommend it.