When the Shattered Dreams box kicks in it checks what you own. If you have only a few items you've not yet got and at least one of them is unique it'll start with a small selection so check how many items you get and if you think it's worth it. As soon as it can't find a unique item to offer it'll be full of a whole heap of repeatable items and they're probably not worth going at unless you have a whole heap of tokens and don't mind paying to go and get more.
Not sure yet, but it looks like they've started looping the themes, so if the boxes are as long-lasting as the Yearbook you can wait until they come back round I think - but who knows except the devs . . .