I didn't know fire emblem had the option to turn that off. One of the few reasons I was always hesitant to get one. I guess when the fire emblem 3ds comes out that it will be less of a stressful time playing. Once I learn the game I will probably play it again with perma death but otherwise some games difficulty can turn people off at the beginning and can deter the advancement of some games popularity while others "dark souls / demon's souls" can actually bring in more. It all depends on how the game plays but at least for the fire emblem style its good to see there is an easier mode.
It hasn't always had the option. This is really only the second game to feature casual mode (the first being the second DS game, which was never released outside of Japan).