One of the advantages of decentralized
network is anti-censorship, like gnutellas and Peekabooty. But I wonder
besides this advantage, what else advantages of gnutella-like network has
or better than client/server structure? In which situations we choose
gnutella instead of c/s structure?

Plenty of things. For instance: Failover service - because no single point of failure. Another one: cost reduction (not speaking in badwidth usage terms), such as media casting.

And so on, and so on.

-- Mike