Well, the news after TUF 12 tryouts were that a lot of promising Middleweights turned up and there just wasn't enough room to fit them all in one season. I could see why'd they request Middleweights once again this season.
They could just be requesting for both divisions just in case the opposite happens this time around. If there aren't enough fighters from one weight class, they could add the other to fill up the space. Welterweight and Middleweight isn't that big of difference, they could just request certain spotlighted fighters to move up or down in weight if they wanted to make the show, and it wouldn't be that difficult. That's what happened to Mike Swick on season one. He went out for Middleweight (his natural weight) and they said if he wanted on the show he would have to fight as Light Heavyweight, and that's what he did. They just ran out of room on the show for that division.
I'd like to see Bader .vs. Jones happen. But, something tells me they might have been holding Wanderlei back for this opportunity, and if that's true I won't complain.