Fullnodes need to be incentivized otherwise they will become a liability to the entire network. Bitcoin cannot rely on the assumption that there will be altruistic users running fullnodes.
How can a full node be a liability to the network? What do you mean ?
*cue movie trailer guy voice*
In a world where full nodes are too expensive for normal people...one government (or company, etc.) masquerades as thousands of "full nodes" that play by their own rules. Will the legitimate users be able to find anything other than the government nodes? Coming THIS SUMMER.
*end*
It's not that honest nodes are a liability. Assuming most/all full nodes will be honest nodes can be a liability, especially if you're an SPV client. Of course, the real trouble only starts when you are connected to 0 honest nodes, only dishonest ones (say, ones that run their own chain, and don't accept your transactions).