For a cryptocurrency to function requires more than just a network of miners. It also requires nodes to propagate messages, serve the blockchain and provide security to the network. Essentially it needs full nodes, which are running 24/7 and have the core client on a machine installed with the complete blockchain.
Having more nodes means a greater decentralization, which means a more secure network so that users can always find a ready peer to propagate transactions. The problem is that the number of full nodes on the Bitcoin network has been dropping, from around 10,000 full nodes last year to around 6,000 today. One of the reasons for this is that there is currently no incentive to run a node, whereas miners get rewarded for their services.
Block Size Debate
This feeds into the block size debate, as bigger blocks could require more resources from full nodes, reducing the interest of hobby and independent operators, and leading to greater centralization as a result.
Gregory Maxwell recently said:
“Do people (other than Mike Hearn; I guess) think a future where everyone depends on a small number of “Google scale” node operations for the system is actually okay?”