Posting in the forums is not completely optional. I mean sure, you can go your entire game-playing life without doing so, but if you ever get a hankering for posting bug reports or wanting more help than GMs can provide in-game, the tech support and bug report forums are now unavailable to you should you wish to never post with your real name.
Also, if I ever stop playing WoW because of the Real ID changes (so far, they're optional, works for me), it's because I hate getting harassed for being female (and my real name is very definitely feminine). Not because I fear what I say in forums or in-game will have a negative impact on my life otherwise. I just don't play a game to be annoyed like that.
I normally don't care about my name being plastered everywhere, as I'm fully aware of how not anonymous the Internet is, but let's just say I don't exactly want Blizzard to go and tell the whole world my real name if I want to post cause those idiot 15 year olds in that retarded guild on my realm would probably go far enough to post IRL details of people they don't like for whatever reason in trade chat and more. I also don't see why that has to be the case, especially when they plan on introducing a big load of new features to help CMs moderate the forums.
There's a difference between keeping that information thinly shielded behind a pseudonym from in-game vs. screaming the name on your billing address for the world to hear.
But.....yeah, I ultimately have a choice. I have the choice to stop playing the game. Works for me.