We have been able to play as elites in matchmaking previously due to their role in the campaign. In Halo 3 they were allies and we could play as either of them cooperitivly in multiplayer matches. Then in Halo Reach we were able to play against them in Invasion matchups and elite slayer. Since it would be unreasonable to have the elites play with the spartans due to advantages and disadvantages that they have.
Since in Halo 4 Elites and the Covenant are not the main enemy then I suspect that we should't play as them due to the fact that they don't have as much relations with the Cheif and UNSC. I will miss playing as them but I think I will get over it eventually. Also I hope they keep the idea that they are much much larger then chief but not as much in reach because I do believe John-117 is taller than anybody on noble team besides Jorge.
Also the place where matchmaking takes place is on a ship called the UNSC Infinity. It is supposably a spartan training facility which is able to mimic different enviroments in somewhat of a simulation. I can not wait to see what 343i does with the forge mode and what they add to make Halo 4 customs and creations even more amazing than reach.