It is all about knowing what u do.
It is not recommended for the above mentioned reason. There is a very real danger in doing it. However, if you know that and you make sure that A/THR remains in SPEED (by selecting V/S before switching off the FD) or better, you just disconnect the A/THR, then it is absolutely safe to fly with one FD only.
It can be useful, the PM would see how the PF is doing. Could be good for practice of hand flying skills.
I am the inventor of a new flight guidance mode, which doesn't have a name yet (we are in the start up stage). It would be like an intermediate layer between FDs ON and FDs OFF flying. The FMA still shows the guidance modes but the bars are not there. They only appear when you deviate from flight parameters beyond a given threshold. Then you follow the bars. This would make actual hands on manual flying practice safer and airlines would be ok with it. It could also work with A/THR, so you would control it manually until it activated in case of excessive deviation.
Am I going to be a millionaire or what?