Where mobile devices like phones and tablets shine is during a commute on a train or bus. How true it is I don't know, but I heard that in Japan many people just don't have a home computer or laptop, since they spend so much time commuting. In most western countries driving is a lot more common, to and from work and home, both of which have more useful machines available.
This is of course the weakness of mass consumer cloud adoption - if you have a machine capable of doing useful work, you already have a machine capable of beating the cloud hands down in every way without the ongoing cost and risk of an internet connection. For sharing large files, collaborative work, desktop publishing (blogs, flickr accounts, youtube), smallish offsite backups, the cloud works great, but even then you need local copies.