With Hunting Grounds and Apocalypse, it is the most widely used Apex cards. At one influence, it is easy to splash in any deck that can make good use of it.
Every Apex, has 3 copies of it. It powerup everything that make Apex work. So, it is an easy decision.
Outside of Apex, it work really well with if you intend to use Heartbeat or Endless Hunger. But also, it can power up Aesop's Pawnshop. With Aesop's Pawnshop, you put Harbinger for one click and then get 3 per turn for 2 turns (clicklessly). That is some serious economy. Which is why it is often seen in decks that use Aesop's Pawnshop. Sometime, you can even power it up further with Reaver, to get credits and draw a card.
But it can power up Environmental Testing and Technical Writer and you can bring it back with Simulchip or with Reclaim. Making it quite valuable if you are going for that kind of deck.
Finally, it is good fodder for SDS Drone Deployment, Degree Mill and when you need to trash something from an ice like Fairchild 3.0.
The card, the color, the image, the baudot code, everything screams Apex and its theme. Downright fantastic!