After asking around on forums with other Cubers and after some serious thought, I've decided to reject the father of all casual cards for the Cube:
Just as Illusionary Mask was the original face-down card enabler, Chaos Orb was the original "Un-" card. It's banned in every format but casual and Cube is a casual format, but I just can't do it. It's just too goofy. It's everything I wanted to avoid when considering any Unglued or Unhinged cards to include in my Cube and it's still hard to resist. (The nostalgia factor is strong in this one.) Alas, I've decided to make the intelligent move and just stick to the single copy of Who/What/When/Where/Why and save myself $60+ in the process. And for the record, I do really enjoy the fact that I have exactly one "Un-" card in the Cube. I figure that if you have more than one, it starts being a novelty and the entire Cube kind of gets labeled as an "Un-Cube." But with only one, that single card is just a hidden gem within a normal Cube of cards.
I suppose now is a good time to talk a little about the "catch-all" removal spells I've included in the Cube.
If you look through the Cube list, almost every single spell falls into one of the main or sub-themes of the Cube. Even when it comes to removal, every removal spell or effect comes off of a creature turning face-up, a tribal card, a split card, a card with cycling, or a card that is at least loosely related to the morph mechanic. There are a few exceptions, mostly for the catch-all removal spells including:
Sure, Beast Within's drawback is accidentally tribal and you could even include other themed cards like Necrotic Sliver and the aforementioned Who/What/When/Where/Why, but regardless, I think their presence is necessary for the Cube. I have a few counter spells with cycling and a couple creatures can accomplish the same, but most of the trouble cards will slip through the cracks. A lot of casual players don't like playing against counters, but they really are necessary to keep the board state from getting out of hand and to stop opposing bombs that might just end what may have been a really good game in a very anti-climactic way. In other words, from a design perspective, I think it's necessary to include at least a few cards that can deal with anything broken, even if you didn't think it was broken -- you're not that smart. At least, I'm not.
There are too many variables and too many options to assume that you're going to outsmart every random person who plays with a deck made with your cards. When they play, they're going to do everything they can do to win. If it's possible, they're going to do it. The catch-all removal spells are in the Cube to save my butt from things I've missed. That being said, Chaos Orb also would have fallen into this category and it's a shame it couldn't make the cut.
(Can you imagine drafting both Chaos Orb and Glissa, the Traitor in the same draft?)