i have seen a lot of this done with firewall devices and vlans. with vlans or mpls, you can make spaghetti without wires, one wheat and one semolina.
oh absolutely. you can use many tools to lop off your fingers, my point was that things like mpls (or vlans) provide a nice other tool to use along with your firewalls and such.
of course you ought not willy-nilly go crazy with this, but... imagine if the 'hr department' were in one contiguous 'VRF' which had a defined set of 2-3 exit points to control access through... while those willy 'engineers' could be stuck in their own ghetto/VRF and have a different set of 2-3 exit points to control.
Expand your network over many locations and in large buildings and ... it can be attractive to run a 2547 network that the company is a 'customer' of, or so I was thinking :)
i have seen people successful with this with mpls and with vlans with non-mpls tunnel tech (e.g. ipsec for the paranoid). i have seen them screw the pooch with both. randy