Since most Steem applications don't have a backend that processes information, post scheduling is a challenge. There is no ability in the blockchain to set your post time in the future (which would be the easiest method to accomplish this). So in order to pull this off, your computer would have to be idling on the page when you wanted the post to happen.

I agree though, it'd be a great feature, I'm just not sure how to implement it effectively without giving you an application to download and run on your computer (which would take a long time to build).