Draft page would probably use more server resources and I am not sure it is that important if admins have limited resources.
It can be useful feature when someone is making longer posts,
I think it is a useful feature not only for the longer post I think it is useful for all kinds of posts. Sometimes even in short posts, you have to align the post by using many bbcodes, and it has happened to me a few times that after writing the post, the tab was closed by mistake, so I had to rearrange the post again. And It is boring and a waste of time, in this case, there is no alternative to drafting.
but it could also be written offline and saved in text document for posting later.
I think someone could even make a simple user-script doing this as alternative.
Before getting the script or downloading offline bbcode software people can use bitcointalk forum they just need to have a topic/post on archived board for testing.
