In my case, it's simply a matter of hardwired habits. See, when I was little, I used to watch my brother play Final Fantasy games on the SNES while waiting for my turn with Donkey Kong Country or whatever I had rented that weekend. When curiosity got the better of me and I decided to finally try the game for myself, I played the way any newbie to the JRPG genre would; I rushed it.
I was playing Final Fantasy II (the first US version of IV). Needless to say, my foolhardy playstyle landed me in hot water. Within a few days of starting my game, I had already hit a roadblock in the form of Milon, the first Elemental Fiend you face in the game. He really isn't difficult to defeat, if you're adequately leveled, and even if you're not, healing items and spells can dole out some serious damage. Of course, I didn't know that. I got stuck. To my inexperienced mind, there was only one solution; "Fight monsters, get strong." And so I fought. I fought and I fought and I fought, until I finally pushed through both Milon fights using only the Fight command (yes, even with Tellah, Palom and Porom, and no, it wasn't to show off. I just didn't know any better).
Long story short, I grind out of habit. It's been a long time since I was inexperienced enough to lose a fight in a Final Fantasy game, but even so I like to play it safe.