A few credible guys have trained with Mentzer back in his prime who say he used more sets than what he wrote in print. Were they warm ups for the heavy work sets? I don't know.
I knew Mike, too. I asked him about the reports of some high-set workouts he and Ray did, like their "squat party nights." I even read him some of the supposed "excuses" he told other folks to justify catching him doing higher volume.
He basically just laughed and said those people were morons who took things out of context. He admitted he and Ray would occasionally do what he called "crazy shit" like 15 sets/bodypart to failure.
But that wasn't his standard routine, and why would he lie? As he used to say (heavily paraphrased, because it has been many years): "People think I just want to make money by saying, 'do less, do just one hard set'; but then, people don't want to hear that -- they think I'm crazy. I'd sell much BETTER if I said, just do a little less than Arnold, but do forced reps, drop sets and rest-pause; the kiddies would think that's more hard-core and my books would sell faster than pancakes. But I don't give a shit how well they really sell. I'm already making a comfortable living and I'm not that greedy."
The reason Mike sometimes did lots of sets to failure was multi-fold as I understood it.
A. He'd overtrain on purpose before a long vacation, during which time he wouldn't lift.
B. Back in the day, he understood that you could overreach for a while and gain, but you'd eventually have to back off. That's something he never really wrote about but we discussed it at length. It was his form of cycling training volume, back before he advocated taking 1-2 wk. layoffs.
C. Mike also thought that beating the piss out of a muscle the day before a photoshoot could be a good thing (severe edema so you wouldn't have to "pump up," plus you were so sore you could feel the muscle contracting to better pose/show them off.
But generally, he told me he rarely did more than two hard sets per exercise. Behind-neck press might be 95x5, 115x5, 185x3, then 225x10. After that, he might go up to 245, get 6 on his own and four forced reps.
People can stamp their feet, whine and complain all they like, but if you really consider those first three real "sets," you might as well keep track of the steps you take upstairs to the weight room, raising your arms over your head, etc. At some point we've gotta draw a line