I imagine they didn't include chat bubbles because they feared it could be used for griefing. As it is crucial that you have a good view of the surroundings and your enemies, it might become a real problem if a dynamic event with ~20 people is a total mess of chattbubbles (either for coordination, just to express excitement or in the extremest case, to troll).
Remember the feedback concerning the overuse of particle effects, and now imagine solid chat bubbles instead...
Edit: I really like commas...