How useful are these sorts of things?
Depends...
I have one and I tried to use it to bend a straight upper part of the trunk on a white pine. It didn't work out. I couldn't bend the it as far as I wanted, and after some time I noticed it was damaging the bark of the branch - it left quite deep dents that took a year to recover after I removed it.
I'm now using the same jack on a branch on a larch, and that is working out better (of course the branch is far more flexible).
Wire would probably not work on the pine; I guess I'll need a bigger jack!
Wire would have worked on the larch - and it would have given me more possibilities to shape the branch the way I want it (but in this case, I don't need that and the jack will do fine.
So, again, it depends. They can be useful, but limited. How useful they are depends on the type of tree, the thickness of the branch and the movement you require.
I have a very straight trunk that is 3/4" to 1" in diameter on some starter material that I would like to add some movement to.
What type of tree is it?
Either way, I would want to wrap the trunk with raffia to mitigate scarring, correct?
Again, that depends. In general it's a good idea though.