Dave, others here have given good answers: Namely, that people who advocate "dig everything" are NOT, in reality, "digging everything", in the true sense of the word. At least, for example, they are passing iron. And they may be passing flitty surface foil, but still call that "digging everything", etc....
But you have to consider too, that a "$99 cheapie machine" is simply not hearing the same things that a $1000 machine is hearing, TO BEGIN WITH. Let me give you a real life example: A friend and I were night hunting a plowed field site, that is known to give up reales and other such period items. Since this was a "relicky" site, neither of us was discriminating out anything. You know, like foil, etc... and up was all dug, as this is the rule for relicky type sites. As such, you might conclude, in a site like this, in your logic: "why have a $1000 machine if you're going to dig all?". However, night after night, I kept spanking his 6000 Di pro with my Explorer. For every one good coin or button he would get, I'd get 2 or 3. As each night ended, he kept trying to understand why I was killing him on goodies. We would count out our targets (anything conductive), and our totals were about even. So not even I could understand why I kept coming in with the better items
Finally we figured out what was subconsciously happening! Since my Explorer has tone ID, I must've been subconsciously favoring clean nice 4-star round signals (copper slag, buttons, coins, etc...). So on the one hand, I didn't purposefully pass anything conductive, no matter how low and no matter how ratty. Yet on the other hand, when targets are abundant, your mind subconsciously tends to hone in on the better sounding ones. He on the other hand, with his 6000 Di pro, had no tone ID. And since it was at night, he was not able to see his needle bounces. The 6000 is more of a beep or don't beep sort of machine (less tell-tale audio than a more expensive machine like the Explorer). So the end result was, I was coming in with the better targets, while he spent much more time chasing .22 shells, tiny shards of foil, etc... However, if you had asked either of us, up till that time, if we were cherry picking at all, we would have each adamantly denied it. It was only a subconscious thing that the user of the more expensive machine can't help doing, when there are ample targets to choose from, and only limited time. Notice too in this illistration, that depth had nothing to do with it, since with plowed fields, old targets can be both deep and shallow, d/t mixing up with plows.
But like I say, in addition to more target info, I bet the more expensive machine simply hears more targets deeper, to begin with. No one, barring maybe the wide open clean beach, will hunt in true all-metal mode (where a user has no way to ID or pass iron). So essentially, everyone hunts in disc. mode. Once you do that, there are distinct differences that more expensive units offer.
