When people ask why I need so much RAM, I just answer honestly: "Because I'm a badass."
I just upgraded my 2011 quad MBP to 16gb of ram and now I find myself wanting 32gb (which is how I found this thread.) I'm a consultant and like having my whole farm on my MBP because there's no telling where I'll be from day to day.
Currently, I'm running the following in Parallels: FreeNAS, Windows Core for Active Directory, SharePoint Server, 2 Clustered SQL Servers, Windows 7 Workstation, ESXi/vSphere 5 with all the components (vCenter Server, etc), NetApp OS Simulator, and another Win2k8 Core Domain Controller.
Needless to say, I can only run a portion of these at any time with my piddley 16gb of ram 😉