Pilsner, your theory is basically correct. The problem is to apply it in the real world. You are correct that the tag does not have to be "imminent" for the rule to come into play; it's just that it's usually hard to tell if the rule has been broken unless a tag is imminent. Your example, of a runner who turns and runs directly into the outfield, would be a case where it could be easily called. But it doesn't ever actually happen that way.
Picture this: if the line between first and second base runs east-west, then in the "Skunk in the outfield" play the runner, by positioning himself well north of the straight line between the bases, must run either in a southwesterly direction (toward second base) or southeasterly (toward first base) as soon as an attempt is made to tag him (imminent or not). Here's the problem: if he's going in a generally easterly or westerly direction, it's really hard to tell if he's got enough "southerly" going on to know if he's outside the "baseline" created between his position and the base. I mean, do the math: if he's midway between first and second, and fifteen feet back of a straight line between the bags, he can head nine feet due east or west before he's left the "baseline" Considering that no ump will be well positioned to guage the exact angle, unless he makes a sudden detour north or south it's gonna be really hard to make this call. The closer he is to either bag, and the more abrupt his departure from a direct line is, the easier it is to make the call.