Maybe it shouldn't be seen as a hard rule but as some heuristic in the back of your mind, to avoid over-abstraction, digression and never-shipped depression. A counter-balancing notion.

And most of the times people mean YAGNI for the first expression of need-vs-implementation. Surely they know there's a lot of chance that one will need more solid abstraction, but it's just to avoid digging too deep into the wrong hole. Somehow it's like http://en.wikipedia.org/wiki/Simulated_annealing . Avoid being stuck in a local maximum.