I agree with you Iguana in the sense that if these pigs were getting a few handfuls of soybeans each day, you probably wouldn't be able to sense that, and to me that would be something I wouldn't want in my pigs feed.
However, if you were to clone a sheep for example, and feed one offspring all conventional, and then let the other free range on grass and forest, and everything else equal, in terms of handling, processing, etc I guarantee you will be able to perceive the difference, whether by taste, texture, appearance, odor, touch, or any combination thereof. And I think it's this second kind of cleanliness troll is talking about. If you're paying attention when dealing with differently reared animals over the years the patterns are easy enough to recognize. A friend and I were just discussing this recently.
I'm in california right now and it's been kind of a bitch to get grassfed meats in my budget, but only because the nature of my visit has prevented me from investing the usual amount of time I'd put in to sourcing quality food. So I ended up buying some conventional meat, I mean the bottom of the barrel for a while for my dog just to keep him fed. I haven't dealt with that kind of meat in a long time. I see when I go in to the supermarket, but haven't handled it much in the last 4 years or so. I immediately noticed a few things. The meat look devitalized, was soft and squishy instead of resilient, had a mildly foul odor, was slimy, and just 'looked' unhealthy. I've noticed this for years with meat, eggs, dairy, fruits, veggies, herbs, there is a recognizeable pattern when something is of quality. Not something that would be easy to convey though, best learned through experience and one's own awareness and curiosity.
And what is at play here is that there is more toxicity, less vitality (literally: antioxidants, vitamins, minerals, etc) so the conventional meat breaks down a lot faster once cut off the animal because the cells of the flesh are less able to sustain and protect themselves from death/starvation/oxidation caused by the loss of blood-borne nutrients. There are other factors too, my grassfed meat tends to be fresher because of the nature of it and it's supply avenues, whereas most conventional meat, like what you'd get from safeway has been dead quite a while before it makes it to the shelf, and had god knows what done to it along the way.