Storing much of humanity (or at least detailed scans and blueprints) seems cheap relative to the resources of the Solar System, but it could be in conflict with things like eliminating threats from humans as quickly as possible, or avoiding other modest pressures in the opposite direction (e.g. concerns about the motives of alien trading partners or stage-managers could also favor eliminating humanity, depending on the estimated distribution of alien motives).
I would expect human DNA, history, and brain-scans to be stored, but would be less confident about experiments with living humans or conscious simulations thereof. The quality-of-life for experimental subjects could be OK, or not so OK, but I would definitely expect the resources available to live long lifespans, sustain relatively large populations, or produce lots of welfare would be far scarcer than in a scenario of human control.
The Butler citation is silly and shouldn't be bothered with. There are far more recent claims that the human brain can do hypercomputation, perhaps due to an immaterial mind or mystery physics that would be hard to duplicate outside of humans for a while, or even forever. Penrose is more recent. Selmer Bringsjord has recently argued that humans can do hypercomputation, so AI will fail (as well that P=NP, he has a whole cluster of out-of-the-computationalist-mainstream ideas). And there are many others arguing for mystical computational powers in human brains.
Seconding Penrose. Depending on how broadly you want to cast your net, you could include a sampling of the anti-AI philosophy of mind literature, including Searle, maybe Ned Block, etc. They may not explicitly argue that AIs would keep humans around because we have some mental properties they lack, but you could use those folks' writings as the basis for such an argument.
In fact, I would be personally opposed to activating an allegedly friendly superintelligence if I thought it might forcibly upload everybody, due to uncertainty about whether consciousness...
As Luke mentioned, I am in the process of writing "Responses to Catastrophic AGI Risk": A journal-bound summary of the AI risk problem, and a taxonomy of the societal proposals (e.g. denial of the risk, no action, legal and economic controls, differential technological development) and AI design proposals (e.g. AI confinement, chaining, Oracle AI, FAI) that have been made.
One of the categories is "They Will Need Us" - claims that AI is no big risk, because AI will always have a need of something that humans have, and that they will therefore preserve us. Currently this section is pretty empty:
But I'm certain that I've heard this claim made more often than in just those two sources. Does anyone remember having seen such arguments somewhere else? While "academically reputable" sources (papers, books) are preferred, blog posts and websites are fine as well.
Note that this claim is distinct from the claim that (due to general economic theory) it's more beneficial for the AIs to trade with us than to destroy us. We already have enough citations for that argument, what we're looking for are arguments saying that destroying humans would mean losing something essentially irreplaceable.