The way I see it, even sans colon "Into Darkness" is a subtitle. It'll look silly if the title card reads "Star Trek Into Darkness" all in one line of text.
So, does it seem silly because "Star Trek" hasn't been phrased in this manner before? I find it has a peculiar ring to it, but that's why I like it. It's different and "Star Trek" is taken in a different context to the usual overarching franchise header.