Not just that. Abholen is used for goods, as well - think of abholbereit, Abholdatum and so on. It's true that for picking up a person, holen alone would be unusual (not impossible, though). For objects, however, there's no difference between holen and abholen.
I doubt very much that "Ich hole morgen Eier" and "Ich hole morgen Eier ab" means the same thing to the average speaker. "Abholen" has also for things the connotation of waiting for you to be picked up.
Ok, you're right. In this example, "Eier abholen" would mean I've ordered them previously; "Eier holen", not necessarily so (if preordered, most people would rather say "ich gehe morgen die Eier holen", though this is only a usage and not a rule).
On the other hand, if I'm ordering eggs or something else, there's no difference whether the merchant asks me "Holen Sie's, oder soll ich's liefern?" or "Holen Sie's ab, oder...?"