Indeed, the Borg presence near Earth doesn't really require any explanation. Why wouldn't they be there? They are everywhere. From the looks of it, they have always been. As per "Dark Frontier", they just don't bother to show themselves to species they deem too primitive for assimilation for the time being.
"Regeneration" as such isn't too objectionable a plotline or a Borg saga chapter, IMHO. What is profoundly silly there is that T'Pol can somehow divine that it takes exactly 200 years for the Borg message to reach its destination. How can she do that when all she has is a direction, and no idea of distance? The message might have been aimed at a target just two minutes away, or outside the galaxy by twenty thousand years. Also somewhat silly is that the Borg never manage to seriously harm our heroes, and that Phlox is successful in fighting the nanoprobes.
Timo Saloniemi