Becasue this is the main way that I share photos with family and friends - I'm usualll the one with my camera whenever we are on hols/day out etc and to share the photos I uploaded them to my MobileMe gallery direct from Aperture and then they can all view them easily from their iPhone/iPad using the MobileMe gallery app.
I know flickr is there but to get the photos full screen on iPhone/iPad its a bit more of a faff than just pressing a few buttons (unless I'm wrong here). This is one use that Apple seems to have seriously neglected in iCloud