""Because in fact at a higher level it is often done"".. But more often not done at any level. If there were a 100 dancers at this very moment doing a Spin Turn I doubt if a dozen would be using Sway on 4 5. Excluding where you live.
It takes the sixth step away from how it should be done. Too much weight will be on the RF to be comfortable. It would be neccessary to quickly change the sway into a neutral position to be able to do the most used exit which is 4 5 6 of a Reverse. But if you are talking about very advanced competition dancers they at the end of a Spin Turn could be anything but 4 5 6 of a Reverse.
It should be mentioned that watching this I doubt if there are many who compete.

"""Because in fact at a higher level it is often done"".. But more often not done at any level. If there were a 100 dancers at this very moment doing a Spin Turn I doubt if a dozen would be using Sway on 4 5."

That just shows how little exposure you get to championship quality dancing.

"It takes the sixth step away from how it should be done."

Don't project your own limitations on to others - we don't suffer from them, nor obviously, do our teachers and role models.

"Too much weight will be on the RF to be comfortable."

"It would be neccessary to quickly change the sway into a neutral position to be able to do the most used exit which is 4 5 6 of a Reverse."

There's plenty of time; however, sway it less likely to be used in spin turns followed by a reverse turn than in overturned ones that continue the flow into another figure still moving down LOD.