I dunno, organic components have some advantages over mechanical components. Mechanical components have the computational speed, ease of programming and maintainability but organic neural networks have the advantage in terms of abstract reasoning and absorption of novel concepts. I can see a fully mechanical race wanting to take advantage of that.
But I always got the impression the Borg started out as an organic species. Although advantage us organics have, we can spring ourselves out of basic amino acids and lipids and all it takes is a few billion years.