I gave a presentation last week with the above title. In my preparation it wavered between 10 reasons to do one, and 10 reasons NOT to do one, which indicates my ambiguous take on MOOCs, so I settled for half and half.
By "do a MOOC" here I mean for an instructor or an institution to offer one, rather than a learner take one, although you can infer some of the learner reasons also. Later in the week I followed the uniteMOOC session up at Newcastle via Twitter and some very similar responses were being given there. My presentation is below, but actually, you'd be better off looking at Sheila MacNeill's splendid Prezi on the subject, which was part of the Newcastle event.