[chef] Memcached on a separate node?

[Warren Bain < >]

We are using the application cookbook to do Rails deployments.

I'm wondering about the logic around memcached which requires a separate node 
that is not the app node to be set up as a memcached master.  I'm not expert 
in this area at all but the common wisdom from our Rails developers is that 
memcached should be on the app server, not a separate vm which would seem to 
defeat the purpose.  In particular we are doing deployments across 
availability zones so the network costs is higher than on a same physical 
host vm.

Can someone in OpsCode explain why separate memcached node is the model?

Regards,
Wazza