Since you can exert a character with a cost of three or more to do it, you can keep your ink open to potentially cast any of the two cards you just drew
There's no question that ETB abilities are some of the most powerful and undercosted things in the game, so having more ways to effectively answer cards like Dockside Extortionist and Thassa's Oracle feels like a net positive
This downside for the casting player results in the cards being extremely powerful for their invested mana cost
Common cards are the most commonly found in booster packs, followed by Uncommon, Rare, Super Rare, and Legendary
Grind over skill One common criticism is that the system rewards the quantity of games played (or grinding) over the quality of play
Essentially, the synergy from keeping a weaker set of two cards needs to overcome the raw power of keeping the two "best" cards to make this the correct thing to do