Correct me if i'm wrong; but i don't think photo stream does video. And if it did, imagine the bandwidth that you would be using if it was on 3G
Also, it doesn't upload it, it just stores a secondary copy, the original video would still be stored on your device and if you were to remove it it would be removed from the cloud too.
And since the carriers are now monitoring usage, you'd hit your monthly allotment faster as well.