Working to failure helps build muscles(hypertrophy), which in turn increases your BMR and if you're still eating the same calories it would speed up weight loss some. I added them in because I thought they were part of the program even knowing they're not now I still do them and I feel it does help me a lot. I typically add them at the end of my workouts, just something to push myself a little more.
That said, I wouldn't do them on rest days. I need to incorperate cardio into my routine just to help my stamina a little but I think I'll be doing cardio a few hours before I actaully lift or maybe do 30 min every morning.