When you raise the tie, you rather quickly increase the tension in it and increase the bending in the rafter. The problem comes down to heavy tension joinery, is difficult, but certainly not impossible, to achieve with traditional joinery and still have a stiff connection. Steel, unfortunately, seems to be the easiest facilitator in such situations.
All else being equal, a straight tie will have less tension in it than one that is not straight; but, depending on how things frame into it the tie, the increase in tension you get may in fact reduce stresses elsewhere in the truss.
I can't seem to find my copy of #74, so, I cannot comment directly on that. I will search again, however.
