Yeah, but dont you understand no one that is a casual bitcoin user is going to be a node? nodes should be people that can run a computer 24/7 and have power of computing and so on and so fort, not the average bitcoin user.
dumb people have terrabit harddrive with a million of futiles files ... (PPS, JPG)
dumb people have a internet connexion without known the speed (specially upload that it can be high !) ...
what the problem to run and open the bitcoin-core client ?
it need nothing after download the (long and) enterly blockchain ... on normal computer.
Most people do not keep their computer on 24/7. Most nodes that QT will connect to will not be able to send the rest of the blockchain to your computer since it was last online very quickly so you will need to wait to spend your bitcoin anytime you start up your computer.
Ideally, in the future, more people will be using their phone to use bitcoin in physical stores and the amount of data required to run a full node on a phone would be expensive (wireless data is much more expensive then home broadband data) and would drain the phone's battery.