The Creative Commons - Attribution (CC-BY) only license grants blanket permission rights for commercial use to any third party downstream. Proponents of CC-BY argue that this will open up the possibility for new commercial services to serve scholarship. This may or may not be; this is a speculative argument at this point. However, if this happens, this opens up the possibility that these new services will be made available on a toll access basis, because none of the CC-BY licenses is specific to works that are free of charge.
This is very similar to the current model for dissemination of scholarship. Scholarly research is largely funded by the public, whether through research grants or university salaries. Scholars must make their work public (publish) in order to continue to receive grants, retain their jobs and advance in their careers. They give away their work to publishers with no strings attached, often signing away all copyright. A few publishers have taken advantage of this system to lock up scholarship for their private profit.
One potential outcome of a CC-BY default for scholarship is a next generation of Elsevier-like toll access services. Many scholars and the public whose work was given away through CC-BY could be unable to afford the latest and best services made possible by their contributions. This is just one of the reasons to give serious thought to this matter before recommending a CC-BY default. For more, please see my Creative Commons and open access critique series.
Thanks to Heather Piwowar for posting an opposing view on google g+ that helped me to work through this argument.