Do you think we do not have enough nodes running on the network? If we do not need more nodes then what would be the problem with having more people running a light wallet? Most nodes (I think) are run on a dedicated server (or VPS) so having more people run a light wallet would likely not decrease the number of nodes significantly. It would also make it easier for people to start using bitcoin as downloading the entire blockchain is a pain especially when you have to wait 2+ days in order to start using something new.