Hi Das,
Well, I've configured all my serverfarms as L3-farms, some of them multible hops away. So far, latency has not been an issue, nor have I felt the need to finetune my probes or spend much time digging into server response time.
I guess it all comes down to your infrastructure and your design scenario, but I would think that directly attached rservers mostly come in handy when deploying your ACE in bridge mode or, for some reason, cannot use client NAT the avoid any possible routing issue.
hth
/Ulrich