the thing is, once the Borg knew the Federation existed, why would they ever stop trying to assimilate them? they were a great addition to the series, but had they not been nerfed, the entire Trek Universe would have every non-Borg species just putting all their resources into hiding from, and surviving the Borg.
to be honest, i think the Borg mentality was used much better in Stargate with the Replicators.