I know this topic has been brought up before, but here are some good arguments for making chat an optional feature:
You don’t get distracting or offensive messages during play. Yes, one could block someone or report abuse, but why even bother with all that if we can just not communicate with our opponents as an option?
You are telling your opponents that you aren’t interested in chatting, as a favor to them. This means, even if you find a way around disabling chat such as Zen mode, an official option would be more considerate, so your opponent isn’t wasting his or her time typing anything. It could be “known” to both players that chat is turned off by someone.
I don’t know all the ins and outs of how to add this code to the site, but perhaps another suggestion is: the option on each game itself could have Chat on or off, the way that we turn Handicap on or off, or turn Rated on or off, etc.