> Why do they need to assimilate whole species?

So then they're gone :)

When the borg is threatened ("threatened") by a race, they choose to effectively end their way of life one way or the other. That is done by assimilation or by death.

If only a single outstanding member of a species becomes a threat, that's a strong signal for their overall potential; therefore everything needs to go, the sooner the better.

I thought the Borg wanted to get better and better? Leaving the species essentially intact, and absorbing a few members every so often gives you much more material to work with than a one time infusion: they are still evolving and changing.