For two reasons:
1) I LOVE TORNADUS.
2) I want Landorus.
Off of the top of my head, I can offer a Naughty Haxorus (lv60), a Mild Kyurem, (78), a Mild Venomoth (66), a Naughty Heracross (50), or a Brave Pinsir. I've got other pokes people might want but they've got natures that aren't that...