How useful are hacker arrays?

My own answer about 1 year later: They're pretty useful because the hack price is relatively low and ramps up very slowly compared to adrenaline boosts bought off the black market, and has a much higher number of uses before it's not profitable.

So the thing about resource prices is that you gotta have a better understanding of how to control production and auto-selling at mid to late game stages (or when you actually get to use a hacker array, around HQ level 3). If you're new to the game like I was back then, spend some time in tutorial missions around the Hacker Array and see if you can understand it better. I still don't have a very good grasp on it, but I think I've got the basics of it figured out.

Some of the more obvious examples:

1. There's no way to get a basic resource on the map, and its price is constantly rising. Lower it down to grow your net profits from buying and transforming it into something else.
2. You're the monopolist on a resource and you want to make it much more expensive for your rivals to buy it from the market. Hack the price way way up and keep producing it fast and send shipment to the colony in exchange for stock shares. Your rivals will want to switch to producing other resources so they can still get some shares.

Basically, each means of manipulating the market is as useful as you know how to apply it. So stay in tutorial school, kids :p