even if idea of just running the client sounds appealing for the clients just think what would any bank say for the such proposition saying think of you:
"I'll be walking around with your credit/debit card in my wallet and why don't you randomly send you some money cause I do think you should..."
On the other hand I guess some less popular coins could use some clients acting like a server and having it up 24/7 so others will be able to connect to the network and sync. I wonder if any of the coin owners are willing to pay for that obviously most likely with the coin in question.
btw someone needs to develop a way for the client(s) to make some blockchain movements in the RAM as starting them and writing byte by byte and not lets say in 256kb chunks just kills any HDD. Do all clients need to have full blockchain around anyways??
EDIT: Didn't know any coins like above mentioned PPCoin, Novacoin, YaCoin actually did that
wow