There should be a revision history for discussion posts for many reasons.
If a student says something against the school behavior standards but then edits it, we need a record of what was said.
If we use "must post before seeing replies" sometimes students will post a blank space to see the replies and then edit their response. Yes we see that editing was done, but we don't know that the blank space thing happened.
As much as we'd like to think this would never happen, right now a teacher can edit a student's post to be a crappy post and then assign a low grade. The student has no record of what the original post said when making a grade appeal.
I feel like this one's a no-brainer and ought to be fixed ASAP.