Nolan and Snyder have talked a lot about making this a more "grounded" world, so having Supes go up against killer robots, giant aliens, or monstrous mutant versions of himself would probably defeat that a bit.
At least for the first movie. I'm sure we'll see some version of those other characters in the sequel.
Which is why the grounded approach sucks ass. As good as Nolan's Batflicks are they were equally disappointing in some regards because of the limitations set by themselves(!).
I hope you're right and they cut loose for sequels.