I love just how personal it is. It makes it that one playthrough of a game doesn't have to be the same if you wish.
I fell out of love with Final Fantasy a while ago, but this is one thing I felt that Tactics and Final Fantasy V did very well. And I think that's why FFV Fiesta is still going strong today. When the game puts you in charge of how you want to handle a battle it then becomes less of a "game" and more of an experience in my opinion. Being able to mix job abilities also is a huge plus in my book. Rather than the game deciding what each character is able to do in their growth, putting that decision in your hands allows you to make God like characters and I think that's the height of enjoyment of games that include these systems.