Quad tracking and double tracking is not some relatively new invention, nor is it mainly confined in metal. It's been going on for a loooong time, and in other genres besides metal...and double tracking, even longer. I was quad tracking rhythm parts on my crappy borrowed Tascam 4-track back in the late 80's, after I first read about people doing it. If that gives you an indication that it's been done for quite a while now. Yes, I'm an old fart...and that's okay.