The only risk to you is that I may have also signed that same bitcoin over to someone else.
More recently another risk emerged: The ability of the node to feed the user a different chain, and then proclaiming that this chain is Bitcoin - when it's not.
In this scenario, a userbase which relies heavily on a few nodes can be switched to any implementation the few node-owners want. So if a few node-owners collude, they can switch nearly the entire BTC userbase to another coin and then proclaim it BTC.
So nodes are important. Very important.
Sounds like you don't understand how PoW works.