So both players get the extra card, right, just like with Howling Mine? Yes, that's true. But some players figured out you could get around that if you had Notion Thief in play! You'd be able to draw the extra card, but your opponent wouldn't, along with any other cards they'd draw outside of their draw step. Also, with Thassa, God of the Sea scrying for you each turn, your draws would likely be better more often. It seemed like Dictate of Kruphix would be a really good card advantage engine.
But as it turned out in Standard, there wasn't really a deck that could use Dictate of Kruphix effectively. Adrian Sullivan of Star City Games tried to build an Enchanted Prison/Turbo-Fog deck. That archetype never really panned out in that format. There was also a Maze's End Turbo-Fog list that tried it out. But that turned out to not go much of anywhere, either.
Some people would later try to build mill decks where forcing your opponents to draw extra cards would speed up your win condition. It would be paired with Jace's Erasure, which would make your opponent mill a card from the top of their deck each time you drew. It seemed like a good idea. But that sort of deck never really went anywhere.
So for the longest time, Dictate of Kruphix didn't really see much in the way of competitive play, not in Standard anyway.
However, Dictate of Kruphix would eventually find itself on the competitive scene, even if mostly on Magic Online. Some Modern lists running Time Warp and Temporal Mastery found a use for 4 copies of Dictate of Kruphix as a way to gain extra card advantage during the additional turns the deck would take. Since each copy of Dictate of Kruphix stacks and grants an extra card, you could draw 3 or 4 or even 5 cards each turn. This Modern Walks deck list and this top Grand Prix Charlotte Turns list proved to be pretty brutal control decks. Finally, Dictate of Kruphix found a home in competitive play. It didn't even need Notion Thief to make it good. It just had to be played alongside a bunch of extra turn spells.
Unsurprisingly, Dictate of Kruphix has found its way into plenty of Commander decks, too. First and foremost, Nekusar the Mindrazer loves to make everyone draw extra cards, especially since that Commander's ability hurts your opponents for each card they draw. But there are plenty of Commander decks that don't mind letting opponents draw extra cards, so the Dictate has plenty of homes in EDH decks, too.
When a full-art promo version was given to Journey Into Nyx Gameday winners, it's not surprising how excited players got about this card. While it didn't work out in competitive play at first, Dictate of Kruphix turned out to be a good, useful card.