Hive species tend to respond... unsocially towards other colonies of the same species. If Diana's/Anna's colony wiped out the rest of their kind on their homeworld, they'd be left with two choices: Risk inbreeding, or learn how to adapt sufficiently similar species for procreational stock.
Due to our inherent humanity and our emotions being practically infectious to the Visitors, something they fear and despise, why risk procreating and creating a new generation of V's with emotions? Because they Vistor's have apparently cherry-picked a few races clean... Earth is the closest viable target in reach left, Earth has the technological development high enough that they can use it, but low enough to not be much of a threat, and they're possibly scraping the barrel in order to get themselves evolutionally prepared to go after some bigger fish.
The skinning of Malik shows that once the V's have human skin put on them, it is permanent, because Ryan reveals that that a V can only survive a half hour after being skinned. This means that the human skin is not only tied into the V's nervous system, there's a good chance it's tied into the circulatory system. As demonstrated by the appearance of Diana, an older V, the skin ages like human skin. Therefore, it would need some blood to keep the skin nourished.
In "Siege", why did the Fifth Column need to use actual kamikaze vests? Why did they need to use actual hostages? Since it was all a ploy to buy some time, they could've just had completely harmless but convincing fake explosives and planted the hostages along with the bombers. Then everything would be alright (though Eli would probably be in custody)..
In the season 2 finale why did Joshua allow Anna's plan to carry through by giving Fake Lisa "enough memories" to mate with Tyler? He gained his memories back and I feel like he would have done something to prevent Anna's plan from being fulfilled.