The upside of on-demand graveyard hate far outweighs the downside of only tapping for colorless mana
As it is random it will be able to hit lands, including basics, which no other cheap discard does
It's a card I've struggled to even buy one copy of for the decks that need it
Why are these cards so expensive
Really I just see this as a combo piece and so it is somewhat narrow, it is not even that powerful compared to many of the possible targets, and certainly not for its mana cost, it is just so highly convenient for combo
The best such example is the APAC Glorious Anthem, which was never given out, but has slowly leaked into the market through other means (supposedly, a former Wizards employee was given one as a parting gift)