The shoutbox is frequently cleared of all messages, it's meant as a method of quick paced communication and not for something that you're going back to later. If that is what you are looking for, I suggest using the forums or comments :)

Staff see past posts to do their job, though.

And, where would be the cutoff point? Half an hour? It's way too much chatter. Ten minutes? Some will say it's not enough

Ah, I see. It's just a slight struggle when you miss a reply or something of that kind, but I guess we could always just ask the person to say it again

I'd say a good ten minutes would be enough, though that's just me. All it's for is basically just in case you miss a reply from somebody, or if somebody posted something important that you didn't see. It's kind of annoying to sit there seeing people say "Wow, that's amazing, so-and-so." and not be able to see what they posted. ;P It's just something that a few of us figured would be a good idea, although it seems like there would be a lot of struggle with it.

This can be locked if there's no point left to it, lol.

Locking at request of OP :)