Digital BW, The Print

I'm very happy with QTRgui...there are *never* any bad surprises and
black is always black, when I want it to be. 

I can from long experience previsualize B&W from color, just as from
the "real world," and I don't mind a couple of test prints while I get
the B&W output I'm after from color on my monitor...(I do get very
accurate color Vs color)...and the QTRgui sliders make it easy get the
kind of tone I like the most from any give paper (I like old Agfa
Portriga Rapid look on Entrada). 

So what advantage is there in working with QTR, rather than with QTRgui?