I know, it's a favorite, especially with the TNG crew. But really, what are the Borg doing? I thought they were interested in Humanity for its technology, but Earth has worthless technology in the 21st century. It's not like the Borg need to eliminate this great Federation threat, which has been able to only destroy 2 cubes (out of thousands?) at a cost of many Starfleet ships. How about an "all-out invasion" constitute more than a SINGLE cube?? Especially since the Borg have a transwarp conduit right on Earth's doorstep (according to "Endgame"). I seem to recall the Voyager episode "Dauntless" where the alien described "hundreds of cubes" just around his single planet, much less an entire Federation. Do the Borg need a source of drones? I doubt they would pick Earth, considering all the drawbacks listed by the Queen in "Dark Frontier," not to mention that BEFORE WWIII would've been a better choice (600 million more drones). Plus I don't think that they're running short on drones.
The only thing I can think of is that the Borg want a foothold in the Alpha Quadrant. But without Earth, all that delicious (or not? not too effective vs. Borg) Starfleet technology doesn't exist, and so they would begin assimilating species with also not-too-impressive technology in that century (save for the Vulcans). The Borg should just send a few hundred cubes straight to Earth and collect all that great 24th-century tech and be done with it. Stop the clever ploys that don't add up.