No, they don't all agree...
You only need some of the current core nodes in your unl to be sure to be part of the main chain, as compared to bitcoin where the Genesis block is simply compiled into the main client.
As ripple uses ledgers, this wouldn't work (you only need about 40 mb to get going, not 10+ GB of data). They could put some core nodes in their code, but then people would again complain that this is too centralized etc.

Your own server isn't useless, it is just a fork if you don't use the main ledger chain.