You are correct about the effective change being half of the adjustment number. Consider that if your rifle shoots a two-inch group, a bullet will hit within ONE inch of your aimpoint (assuming your group centers on your POA).
Since the likelihood is that the next bullet will always land within half the gun's average group size, it makes little sense to have scope adjustments larger than that (1/2 the group size). And likewise, it makes little sense to have scope adjustments smaller than half the group size, because group dispersion will be twice that much, anyway.
That being true, unless a rifle is shooting groups consistently smaller than 1/4 MOA, it simply cannot make use of scope adjustments as small as 1/8 MOA. Even if you put in two clicks, you'd never know if the next shot will move at all, because your group size is larger than the correction.
