I'm not getting the question. There is no 'reply all' in threads.
There is 'reply' which, i'm not sure what the point of is. Maybe it alerts the person that they have been replied to if they have such things on? Just a guess
There is 'reply with quote' which just quotes the person making it easier for everyone to know who you're talking to and what you're responding to.
And 'multiquote' for quoting more than one post at a time.
'Reply all' is for private messages sent to multiple users.