The idea behind Harbinger is that it can be trashed twice by cards that need installed card trashing to fuel their abilities, like Endless Hunger: the first time you turn it facedown, the second time it goes to your heap.

You also want to make sure you have a Harbinger out before you run with Endless Hunger on the board, if you're anticipating an X=0 Power Shutdown.

Does Harbinger work with Faust? This is the same sort of thing that Tri-maf Contact deals with, and in that case, if you trash the Tri-maf from your hand then you don't take the damage since it isn't in play beforehand. Precedent says that the same thing would happen here: Harbinger gets trashed from grip and goes straight to heap. —
Harbinger does NOT work with Faust. Unless the card effect necessitates it (like I've Had Worse) cards in players' hands are not active. —
Sorry for the confusion on Faust. I've updated my review to specify "installed card trashing" and not mention Faust. —
Is it still a 0 cost card for power shutdown when its face down? —
No, it would no longer be eligible as a target for Power Shutdown since it is neither a program nor hardware when facedown. A flipped Harbinger will not protect your Endless Hunger. —

So, bear with me while I Jank God out for a minute. If you have Aesops pawnshop in play, plus a couple Reavers, and an Assimilator, you can play and trash a faceup Harbinger, bringing it back into play facedown. Then use two clicks to trigger assimilator, flipping it back faceup, and simply repeat the process over again ad infinitum. So in theory, once set up, you could be pulling 3 credits and 2-3 card draws for the price of two clicks anytime you want.