Well I can’t copy paste his whole chapter on the matter, Popular Biomechanics, but I’ll quote “Even if you wanted your back vertical due to the mistaken idea that a vertical back is “safer” than an inclined back, ultimately it doesn’t matter because your skeleton cannot pull a heavy weight this way.” And further in his chapter A New, Rather Long, Analysis of the Deadlift, he talks about how a vertical back compresses the spine, and in Deadlifts that this exercise is the best exercise for the back.
So I think his position is clear, and then I would surmise that you’re simply not giving the back the healthy workout it would get from a normal deadlift.
I have no skin in this game as I’m not buying a trap bar, I just happen to be reading this book recommended by Chad when I saw this thread.