I think it's only smart to include it because obviously, it's setting up for Superman to be the next big thing. The pace Superman is going, before having a hookline for a sequel at the end, they are going to have to be firing on ALL cylinders throughout the movie to keep the fans interested for a climactic ending.
If they put out Man of Steel to what people are expecting for it to be and if they hit it like they did Batman Begins, then I can see the hook-line for the sequel being quick, to the point and leave the audience wondering with excitement.
IMHO, Brainiac is INEVITABLE for a sequel, it's time for Brainiac for the big screen.
Maybe have the ending do something like Superman being obviously bothered
by Alien and Kryptonian-like language and interference through his Super
hearing and the screen cut to one of Earth's satellites out in outer space
orbiting Earth and then out of nowhere have Brainiac's ship with the
intimidating tentacles slowly move by one of the satellites in an eerie fashion
then end credits. Quick, easy but effective.