Mmmm....
Always an interesting topic this one.
I think the real danger with 1 handing comes from not knowing when it is really dangerous. I've seen climbers put themself at huge risk doing this without knowing it, thinking all is fine. To me, there's an acceptable time to do it and and an unacceptable time to do it. (One is when worksafe come by
)
Seriously, cutting a branch you dont need to hold is usually ok. I think cutting a left facing lateral holding the branch with your left hand and cutting with the right or a total vice versa of this is usually ok.
Its when you cross over, or cut vertical branches/stems that you face the massively increased risk of cutting yourself when the falling weight of a branch maybe in the wrong direction drags your arm across the saw.
This is the time to either rope it, step cut/snap/throw it, or sometimes i attach a sling and hold it to bring my arm/hand well away from the saw.
But those that just cut away at anything with 1 hand just coz ya can with a tophandle saw are asking for it. These people are the ones behind the injuries that are so common in this industry. If you work like this its just a matter of time before it happens.