You make some fair points. As far as Sybil attacks go, an entity could create hundreds of RaiBlocks nodes on a single machine; however, since the voting system is weighted based on account balance, adding extra nodes in to the network will not gain an attacker extra votes. Therefore there is no advantage to be gained via a Sybil attack. As for spam protection, in the article you'll notice that it mentions there is a minimal PoW in order to prevent this. Moving on, when I spoke to the devs, I asked them about the incentives for running a full node. Considering they're evangelists they believe that people will set up their own nodes to strengthen an open system, which has been happening. Exchanges also have to set up their own full nodes. I believe that these are very minor flaws that are currently being dealt with as it becomes more adopted by the community at large.