The Borg Farm Theory is probably the dominant paradigm in Borg behaviour analysis, but it certainly cannot be the only theory. For some reason it just feels too easy for me, which is probably because it makes sense but still.

Some alternatives:

• The Borg overestimate their own abilities, and believed that a single cube could take on the Federation (as observed at Wolf 359 they aren't that far off). Their lack of more resource investment in the assimilation of alpha quadrant powers under this theory would be due to them being a less juicy target - not worthy of multiple cubes, especially if they did not always have that transwarp conduit straight to Earth (and if they did, it would mean no battle with a large portion of the fleet, less assimilation).
• Assimilation requires a massive investment of resources, to defend the site and assimilate the populace simultaneously. This requires many cubes, the Borg were happy just to take an appetizer until they feel it would be a good investment to get the whole meal. This second part is similar to the farming theory except that it would assign the Borg less strategic ability.
• The change in Borg aims (colony capture vs assimilation) and communication style throughout the series (Faceless, Locutus, the Queen) could indicate a fast change in 'personality' as a consequence of new assimilations. They might still be in a process of evolution with the processing of new information. This would contradict the farm theory's belief that it was primarily a strategy to frighten the natives, which I find doubtful considering they've been seen doing it to full planets in their first appearance.
• EDIT: Also, being concerned about overloading with new personalities, leaving the Borg's core ideology to fluctuate could be a risk if they assimilated the Federation or any massive population.

Any other suggestions?