Amongst other things, LNAV won't arm with the FD off. Ask me how I know ......
But that can be fixed.
The only problem is you risk being flayed alive at some airlines if you dare to switch off the flight director for whatever reason.
That's because those air;liens never bothered to train their crews in raw piloting skills, and when some enthusiastic adventurous pilots try it on the line and make G/A's due to unstable approaches the bean counters and nervous CP's step in and ban it. As trainers we are always taught to find the root cause of a problem and start the treatment/cure there. Here he root cause is lack of skill & training. Treatment? a sticky plaster = FD + AT. Root cause still exists and is a latent ticking volcano. Every few years it spews out some ash & fumes. Once in a while it barks and shoots some dragon's fire; one day it really lets go. The same is true with minor incidents, major incidents, and a crash. The signs have been there all along.