In The Raven, its mentioned that the Borg know much of the Talaxians because a small vessel was assimilated and their characteristics were added to the collective.
Why do the Borg then need to assimilate any more beings once they get a few? It would seem inefficient to assimilate more and more beings that don't add any new data or optimizations.
We are the Borg. Lower your shields and surrender your ships. We will add your biological and technological distinctiveness to our own. Your culture will adapt to service us. Resistance is futile.
Even though they knew "much of the Talaxians", they wouldn't have a very large sample size of biological distinctiveness. Even though most members of a species seem just about the same, they are not. When Picard is assimilated by the Borg for instance, the Federation becomes very worried because the Borg would contain his memories and intelligence of Starfleet tactics etc.
Plus, you can always use more minions.
In a way, it's a tactic to eliminate threats at the same time as increasing their own strength. There is a very clear explanation directly from the Borg Queen in Dark Frontier while Seven of Nine was held prisoner. The relevant quote:
SEVEN: Neither. There are only four life forms and the vessel is heavily damaged. It would be an inefficient use of our resources. We should ignore it.
QUEEN: In this case our thoughts are not one. If those individuals are allowed to survive, species one zero zero two six will survive and continue to resist us. But that's what you were hoping for, wasn't it. You tried to mask their life signs but I detected them.
So, a defenceless weak ship that wouldn't actually be worth the resources required to capture is still going to be assimilated in order to prevent potential future threats. They could just as easily have destroyed the ship but getting a handful new drones is better than wasting energy for weapon's fire, I suppose.
The only other way that Borg could add to their collective is by maturing embryos, which is a slow process. Assimilating a world isn't just about adding technology or biology to their strengths.
Further to that, exceptional individuals can only be found by assimilating legend groups. And they may need to assimilate an entire world to find a La Forge, Picard or Data.
It's worth noting that they don't always assimilate entire species.
The best example of this is Guinan's race, the El-Aurians. The Borg assimilated their world(s), and effectively shattered their species's civilization, but there are still El-Aurians in the universe. They seem to have been reduced to individuals roaming the galaxy in solitude, but I can't remember any mention of the Borg "hunting down" the last survivors. It would seem that the Borg won the battle, took what they wanted, and didn't care about the stragglers.
Similarly, the Borg tried twice to assimilate the Federation (at the Battle of Wolf-359 and again at the Battle of Sector 001), but seem to have put little further effort into the endeavor. Two cubes destroyed in the pursuit of assimilating a distant civilization, (and Q makes it clear in "Q Who?" that Federation tech isn't terribly impressive or attractive to the Borg), and it seems to have become more trouble than it's worth.
And, while its canon status seems somewhat debatable, the Borg even seem to have scooped up a few Romulan colonies before (apparently) deciding that Romulan tech wasn't worth even a first assault.
When the Borg decide that your technology or culture is worth assimilating, they come at you hard and they take everything. But if you don't have anything they want, or if they've taken everything from you and turned you into space-gypsies, they don't seem to care what happens to you.
