They are more color-intensive, so they can only be cast in a limited variety of Commander decks, but in return, they can destroy a wider variety of targets than mono-colored removal spells can
Yeah, its easy to see why Crop Rotation has floated around on ban lists for quite some time
WotCs R&D always receives questions about when youre going to complete said cycle. As an example, we can cite the 1-mana tutors that tutor for a card and put it on top, like Vampiric Tutor, Worldly Tutor, Enlightened Tutor, and Mystical Tutor, lacking the red one
Treasures are useful because they can accelerate your mana in a pinch, or you can use Magdas payoff effect to find expensive artifacts or Dragons to close the game with, opening up a cool token play pattern that is a breath of fresh air after years of Krenko and Purphoros, God of the Forge combo killing tables by generating a zillion goblins
This could allow you to Exile all of your opponents permanents with Ulamog, the Ceaseless Hunger, for example