Training MMA makes sense for NFL players in the off-season. After Charles Tillman on Chicago started crediting his MMA training for his outstanding ability to force fumbles, I figured more players would start doing this. Plus guys like Mitrione and Shawn Jordan showed that big athletic football players can get to the UFC and have success very early. A lot of NFL players are out of football well before their 30th birthday, which gives you plenty of time to make a move to MMA.
On a side note, can you imagine how crazy the UFC HW division would be if they could attract the top athletes out there. Everyone knows the best HW sized athletes in the US are in the NFL and NBA, but what if the younger kids in the future get into MMA. HW is my least favorite division, other than maybe FlyW, but if they had athletes like Lebron James, Dwight Howard, Julius Peppers, Calvin Johnson, Ndamukong Suh, or Adrian Peterson, it would get much more exciting.