The other major concern would be that you are moving your trust in the security of your data from very large companies that have staff in place to maintain such security as well as an established track record of offering service in the wild to a much smaller company with much less of a track record.
I understand this argument for iCloud, but Dropbox does not have a history of strong security [0] [1] [2]. This doesn't mean that AgileBits is more trustworthy, but it makes sense that they'd prefer to build their own cloud service over relying on Dropbox for the security of their customers' data.
