One of the Fire Emblem series' chief characteristics is the brutal manner in which you can straight up lose a party member forever. That permanence plays a big role in the strategy-RPG's design, so when Casual mode was initially suggested for Fire Emblem: Awakening, director Genki Yokota said "no way."
While some may see it as taking away from the experience, Casual mode is also a great tool for bringing in new players who were previously intimidated. Yokota was angry at the notion, though, and his reaction was explained during a Nintendo developer interview.
"Personally, the first time I heard about [the idea of Casual mode], I was like "no way." My boss brought up the idea, and just like Higuchi-san, I was angry at the thought of going too casual. But in the end, we figured giving players a choice would help expand the appeal of the game. Plus, with Awakening, there are a lot of good characters -- you can see how some people would want a mode where they could keep all of them alive and not have to worry about death all the time."
Yokota even admitted to playing exclusively in Casual mode now, but added that it does take some of the inherent fun out of Fire Emblem. As for future games, Yokota is "pretty sure" they'll be keeping Casual mode as an option.
Be sure to read the full interview for even more insight into the thought process that went into developing Fire Emblem: Awakening, and the decision to run with Casual mode.