XP is tied to your Spartan-IV. Is your Spartan-IV running around in the Campaign? No, that's Master Chief - Thus no experience. Is your Spartan-IV running around in Infinity? Yes, thus experience.
In Reach, Noble 6 ran around in the Campaign, thus gaining experience since you actually played as Noble 6 in multiplayer. It made sense.
I'm just screwing around with answer, don't take me too seriously. --- Series that need to be remastered, added on to, or imported: Time Splitters (Multi), Jump! _ Stars (3DS?), Dissidia (PS3), Monster Hunter Frontier Online (360)
But they gave you xp during the Reach campaign. If fact it was the fastest way to earn xp. They should have brought it back.
your not playing as your spartan IV so you dont get XP it wouldent make sense if you were playing as MC and got XP sent to your custom spartan without your Spartan doing a thing. MP has a story to. It made sense for reach because you played as your custom Spartan III. It doesn't for halo 4 because in the campaign you play as MC. Its the same as halo 3 you didn't get Xp from campaign for multiplayer. You see what im saying? Halo 4 is all story based. --- GT- Secondary Hero