Having been printed in Worldwake, as a pre-release promo, Commander 2011, and Modern Masters 2015, it’s pretty much a bulk mythic rare. Yet, it’s usually on plenty of buylists for more than a quarter - the usual bulk mythic rare. So where’s the demand coming from if the card’s so bad?
Comet Storm is basically a functionally better Fireball, one of the iconic burn spells of Magic. Whereas Fireball divides the damage among targets, Comet Storm deals X damage to each target. That sounds pretty good, right? Plenty of Commander players think so. With so many ways to copy big spells in the format, such as Increasing Vengeance, Reiterate, and Reverberate, it can serve as a finisher in a format otherwise flooded by mana. It’s also at Instant speed, meaning you can unleash it on anyone’s turn.
Wort the Raidmother is a popular Red/Green Goblin Commander that not only brings a couple of Goblins into play when she enters, but also gives your red and green instant or sorcery spells conspire. Conspire is a really cool mechanic that allows you to simply tap two creatures that share a color with the spell that you are casting; you then get to copy it and are also able to choose new targets for it. A massive Comet Storm or Banefire is often the endgame for this really powerful Goblin Commander and her army.
Gisela, Blade of Goldnight is one of the most popular Angel Commanders for good reason: she doubles all of the damage that your opponents receive (yes, this includes the damage your other opponents would do to each other!) Comet Storm is pretty nasty in this deck, even though you would rarely play copy spells in the deck.
Riku of Two Reflections can copy any instant or sorcery spell for only UR (Blue/Red). Two mana to copy a Comet Storm is pretty sweet.
Other Commanders that like to include Comet Storm among their 99 are: Ashling the Pilgrim, Chandra, Fire of Kaladesh, Daretti, Scrap Savant, Heartless Hidetsugu, Jaya Ballard, Task Mage, Rakka Mar, Urabrask the Hidden, and others.
So next time someone hates on Comet Storm, remind them that Wizards knows how good this card is in Commander. Why it was included in Modern Masters 2015 is beyond me, but it’s far from the worst mythic rare ever printed.