Not long after I posted my view on things, John posted his reply on the ChemSpider/OpenData discussion. His comment was merely to illustrate an internal advice to some organization, which got accidentally leaked. Anyway, a must read, with two good links to further reading on open data licensing.
His blog mentions the concept of public domain, where data might be dumped, but I always understood that the US public domain concept is different from that of mainland-EU, German law in particular. This second 'good link' points to a license which formalizes this 'public domain' idea. And reading it, I realize that I have read it before. But I had completely forgot about it.
A quick reread of these two links, tells me that it indeed is BSD-versus-GPL all over again; with the Science Commons license on the BSD side, and CC-BY-SA at the GPL side. The first surely makes the life easier of aggregators who wish to combine licenses. Can't argue with that.
Then again... what's wrong with a bit of viral character in the license? What's wrong with the statement that 'you may use my data, if I may use your aggregated data with the same license'? That limits your what you practically can do, but does not limit your freedoms.