Benefits to private networks between virtual machines on an ESXi host?

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http://serverfault.com – I'm planning this development environment with a few database servers, and originally thought I would have a few private networks. I then thought it might be unnecessary as the ESXi cluster already provides redundancy with 4 NICs (in my case) and should manage the network traffic pretty intelligently, right? Two private networks Zero private networks What are the advantages/disadvantages between the two shown configurations - on an ESXi 4.1 host? (HowTos)