Because the Xi'an/Xian uses its flaps for the vectoring angle, it really needs a way to tell which way the 'flaps# are pointing.
If you're flying without extra instruments/hud, it's impossible to know if your 'flaps' are angled or facing down unless you scroll through the action menu - pretty tricky when landing!
Also, since you can't change to max/min angle unless at very high/low speeds, you can't just press the flaps key and know it will change. If you're not at the right speed, pressing the button might not be doing anything (but you can't tell since there's no indicator).