the f5 many people i think would love to play minecraft with a third person view, but the problem is that when you are in the third person view your crosshair is in your head which means that you can't see what you are aiming at with (a bow - a sword - clicking a botton / lever - where you place blocks and other things)
for me that's important to be change like other 3rd person view games (most of them are war games)