If you send a text to multiple iMessage users, anyone who responds inline will have their message sent to everyone. I previously thought this was an issue with people having “Group Messaging” enabled in Settings→Messages, but apparently this functionality comes standard with iMessage.
He brings up a couple of good reasons why this is a less that ideal situation. I haven’t noticed this personally, but that likely has to do with the fact that I never am involved in multi-party messages like this.
The second point he brings up on privacy though is something that I have never thought of, and is in the same vein of why I hate people that send mass emails with the recipients all in the “to” field instead of BCC’d.