I am entirely behind this, however...
There are other reasons 343 would want to be careful with playable flood are around age ratings as well: the flood can cause ratings to rise so they're always been careful
Would love to see playable flood though.
They already did that when Awakening the Nightmare dropped. That Proto-Gravemind has got to be the most disgusting thing I've seen in a long time, and that Reveal Blur cinematic already shows us a horribly disfigured alien who has... what? Bones? Intestine bits? Coming out of the eye.
EDIT: Plus, Halo is an M rated series in which the Flood is a major reason. If anything, the Flood shouldn't be left out due to age ratings.