No, if you produced slag as a waste product of smelting steel, you didn't make the slag for a purpose. The statement I was responding to was that Aginor created the Myrddraal explicitly for some purpose. In reality, they were an accident that took him completely by surprise. That's pretty much the opposite of "explicitly for a purpose." There is also no evidence anywhere in the books that the Dark One created them explicitly for a purpose either. Whether through Aginor's actions or not. He certainly made extensive use of them, but there's no evidence he planned their existence. And though the Forsaken were only permitted to use the True Power with permission, they were absolutely familiar with it. If it were obvious that's the source of the Myrddraals' abilities, he would have discovered it when he tested several of them to destruction. And no, that's not what "Distinction without a difference" means.

Actually, neither of these things are true. Aginor made Trollocs. But the Myrddraal were a surprise offshoot descended from them. A sort of throwback to the human side of the human/animal mix of the trollocs. Aginor himself didn't know how they came to be, or how their powers actually work. Though not for lack of trying to find out.

I would have loved to see Hurin, Juilin Sandar and Elyas Machera have an adventure together.

It's certainly possible that Rand benefited from skills gained in these different lives. We do know from what other characters said after the Portal Stone experience that they also lived other lives. Not just Mat and Perrin, but also Ingtar and Verin. But by far the most valuable thing Rand gained wasn't skill with the sword, or in channeling, or even in being a leader. It was the absolutely certain knowledge that - in thousands of lives lived - he had never given in to the Dark One. Absolutely certain, and absolutely essential knowledge later in the story.