Maro (Mark Rosewater), the Head Designer for MTG, frequently answers players on his personal blog
In a recent conversation, he defended Magic's mana system for it's ability to act as an "scapegoat" in frustating moments
Image Credits: Magic the Gathering
Cards Realm
"One of the mana systems best qualities is the ability for it to act as a scapegoat. The saying goes “When you win at Magic, it’s skill, and when you lose, it’s luck.”
Image Credits: Magic the Gathering
Cards Realm
Maro also said that the variance allows players to imagine the scenario where they'd have won.