I think there's a real concern on Twitter's end with the conversation being dominated by those who are "good with Twitter". That's a big problem because it's kind of a snoozefest to read tweets by a small in-group that you don't know and that won't interact with you. Twitter serves no purpose without interaction.
I understand the motivation behind allowing users to lock replies, but you don't need Twitter for that type of content. Twitter sucks for just about anything other than interaction.