Spoilers ahead for those not having played GoW1 and 2.
I know they've been hinting that Emergence Day was about the Grubs running from something in the deep earth and wanting to get rid of humanity quickly so they could turn around and face a larger threat, but I can't help being reminded of C&C3 ("Oh noes, GDI and Nod must work together!"), Halo ("Oh noes, Humanity and The Covenant must work together!"), or Warcraft 3 ("Oh noes, the Alliance and the Horde must work together!").
At some point, villains need to stay villains. Also, worm aliens are not nearly as cool in CGI as they might seem on paper.