Its really no different than people dropping to 2 Smashing Ground when you could use 3. Sometimes, you need that one spot for something else, and since Cyber Dragon is more or less a flexible summon (use if to Tribute Summon a Monarch or another 5/6 Star Monster), and now that Ring of Destruction is gone, the chance to move something else into your Deck while still having the flexibility of 2 Cyber Dragon's, keeps you from having to increase your Deck Count to 41 or 42.
Lots of Decks still use 3, and probably will with the decrease of Monarch Tribute Support Material.