Without reading through nine pages, I'm sure plenty of people have said the danger of the combo depends on what your tolerance to each is and if you know what you're doing -- and I would say the same.
That being said, for someone who doesn't have a tolerance to one or either, and isn't entirely experienced with one or either, it can be very dangerous. At about 19 years old, before I had a huge opiate tolerance, I ran into a scary situation where a coworker of mine gave me his 75mg take-home methdone dose and I gave him some Valium, bad things happened, and in the heat of the moment I ingested about 200 mg of Valium with the methadone I had taken. I don't even remember getting home that day (a 45 minute drive), and the next thing I knew I woke up being dragged into my girlfriend's mom's car to go to the ER. I probably would have lived but I had lost basically all function of my limbs and couldn't speak. It was pretty scary as that was the first close call I'd ever had.
Years later I had an even closer call with Valium and alcohol, which is definitely more dangerous of a combo. That time I stopped breathing completely and was kept alive on a ventilator for three days. But that's a whole other story...
I've used benzos and opiates together many, many times since and have had no problem, since I'm very familiar with how much of each I can handle, even taken together. Still, it's certainly not recommended...