I think the problem with some theories here is that it assumes the Borg are capable of long game assimilation tactics, & while that is possibly true, it's also just as likely that they are a pretty unique development by design, and in order to keep it functioning, they might not have the luxury of holding off. When they come upon someone within the proper parameters, they really have to consume them ASAP to not only continue to establish their stronghold, but also to keep the thing operational
Basically, they probably don't think in terms of "Desirability" as @Spot261 was suggesting, but more probably have to view things in terms of viability. If they're viable, they get taken, & the ones that aren't viable, are likely deemed so because not only do they not possess any enriching qualities, but they also present a potential for retrogression.
"We will add your distinctiveness to our own" That might work both ways, & therein it becomes pretty important to walk a fine line of keeping the ranks growing, as much as possible, but not to a detriment
Basically, they probably don't think in terms of "Desirability" as @Spot261 was suggesting, but more probably have to view things in terms of viability. If they're viable, they get taken, & the ones that aren't viable, are likely deemed so because not only do they not possess any enriching qualities, but they also present a potential for retrogression.
"We will add your distinctiveness to our own" That might work both ways, & therein it becomes pretty important to walk a fine line of keeping the ranks growing, as much as possible, but not to a detriment