Was thinking, I backup my iPhone and iPad to my iCloud already, why can't we take Time Machine into the cloud and have an iCloud backup of our Macs? I'd surely like to utilize some more of my 1TB iCloud storage.
It would take ages.
A phone and iPad is different. If all your media is purchased through iTunes then backing it up is not necessary, as it could simply be redownloaded through iTunes store. The OS is available through iTunes, same goes for apps. Basically the only thing being backed up is contacts and preferences, pictures, and a few docs here and there.
A computer? The files are much larger, and there's a whole lot more of them.
At internet speeds most people are getting, it's just not possible to do in a reasonable amount of time.