Also agree upon this note. As I started Halo 4 Infinity multiplayer, I noticed that there was no Team SWAT. I was bewildered. All of their past games have included it, and SWAT happens to be one of my most favorite game modes in Halo. I went through to the custom games and it was under slayer. So it could be a possibilty that it's simply a choice in infinity multiplayer in Slayer variant game types.
But, I wouldn't want just a random chance at the game type. They really should add all of these game types into their matchmaking playlists. I hope they do.