In addition to being an atmospheric visual effect, having dynamic infestation gives us opportunities to develop some interesting game play elements. High on our list is having map entities which are triggered when an area becomes infested or uninfested.
For example, you could have a computer console that shuts down when overrun by infestation and causes the lights in the room to go out. If the marines clear out the room and fight back the infestation, the system comes back online and the lights flicker on to fill the room. What about abilities and technology which only function when players are on their home turf?
Who knew bacteria could be so cool? It's little features like these that will eventually propel us to games that feel, look, and act real.