If Wizards of the Coast decides to print must-have 1 cost legendaries in the next set, it might be time to get a Mox Amber of your own
He'd get the card, and you would be left out in the cold
As pointed out in the comments, its important to remember that The Coffin only gives you protection from everything, not your bits and baubles, in an Indiana Jones hiding in a fridge sort of way
Not completely unheard of, but you probably won't want Ur-Dragon as the head of such a deck
If you have a lot of relevant permanents such that their Detention Spheres always have targets but yours might rarely have non Detention Sphere targets, you might need Banishing Light so that you can remove their Detention Spheres with a card that would otherwise be dead, having no other target than Detention Sphere
It also includes a 2-card Box Topper