We all have probably done it. Posted something that has already been brought up in another thread or someone gives you a logical reason why it wouldn't work so the idea is abandoned but it is still taking up space in the forums. You can delete your own post but not the thread itself.
I think, there is no need to remove these threads. Keep them for further reference and provide a link if there is already a thread about it. And I'm sure, space won't be a problem within a forum. I don't think you should delete posts at all. (except for rare moderation reasons) Edit them if necessary.
