Could you explain why you think that users don't need to run network nodes?
I don't agree, but I would like to understand your point of view.
The design of Bitcoin is such that users can just be users.
For example, if I transfer a coin to you, you can immediately verify my signature. You do not need to trust me or trust anyone else -- the transaction itself contains cryptographic proof that I transferred ownership of my coin to you. You can even follow the chain of signatures backwards and confirm that the person who I received the coin from also signed the transfer, and so on and so forth, as far back as you want to go. You do not need a network node to do this!
The only risk
to you is that I may have also signed that same bitcoin over to someone else. This is the double-spend problem that had not been solved until Satoshi. Satoshi's revolutionary solution was to use proof-of-work to time stamp transactions into a chain such that -- if his proof-of-work conjecture held -- that the same coin could not be spent twice.
As a user then, the only additional piece of information you need, is whether the transaction I gave to you was accepted into the blockchain. A network node can provide you this proof (that you can trustlessly verify for yourself!) with a few kilobytes of information.
Here is a talk I gave (only about 10 min long) that explains how SPV works in more detail (sorry about the crappy audio!)
https://youtu.be/m7cvPvtGIUI?t=459Thanks, I appreciate your response, but you talk to me about just one function of the node, which is to keep a record of the transaction history. Is not it also the function of the node to enforce the set of rules that maintain consensus in the network?