And a horse can get very good at going through the motions. And in many cases, it isn't really a problem. But if you see the difference between a horse that is "there" mentally and one that is going along with things but is not really there, it's dramatic. My trainer (here I go quoting her again. I am as bad as bsms and his constant quoting of VS Littenaur) any . . . She says, when the horse is with you , he is "available". That availability is amazing, and thus you can get the horse to do things with a feather's weight of pressure. I can imagine that horses that are at a rescue or at a school no longer see much purpose in being available, because no one really requires it of them. Heck, most of us don't require it of our horses and we manage, and our horses manage. But a lot more can be done if your horse is "available".
If they eyes are elsewhere, it's likely that he is not available, so to me, there is some truth in looking to see what the eyes can tell you, though if you watch them nonstop, you will miss all the rest, whcih is also important.