Part of it has to do with the time the books were written, and the fact that these never correspond with how the suggested reading pattern should take place. Patternmaster, the final book in the collection, was published in 1976, and is the oldest book there. It feels it, too, especially after the poorly titled Clay's Ark, which is the best book in the collection. Butler does a neat structure wherein she places the present of the story, that involving Blake and his daughters being captured by a group of men and women who plan to infect them, against the past, which details the arrival of Eli, the first man to come into contact with the virus. It's a nice little braiding technique. However, the book isn't without faults: the end is a rushed mash of violence that doesn't really suit Butler's style or tone, and some of the relationships never gel, but it remains the best book in the collection. Wild Seed hits pretty close to it, but there's a four year difference between the two--Wild Seed was published in 1980 and Clay's Ark in 84--and you can just feel the level of difference in Butler's skills and focus.
The real problem, at least for me, is that there really doesn't feel as if Butler had any plan with the novels to weld them into a whole, and as thus, they never really come together as a series. It'd be curious to see what Butler's plans for it were, if she had any; maybe there's an interview out there, but I'll have to track it down a bit later. As they stand, however, there are interesting points to be made about power and ownership, and you can see the themes that are dominant in Butler's work, especially in the two eldest books, but books like Mind of My Mind (she never really had good titles) and the already mentioned Patternmaster are fairly week, the novels of an author at the start of her body of work, and as such don't resonate as strongly as the others. They are, to be honest, curiosities, things to read to see Butler's evolution, but of works within their own right, she produced stronger novels.
Not, mind you, that there's anything wrong with the former.