Hmm - who said they had to be physically separate? Being a layer 2 centric
network has allowed us to extend device management functionality to not only
the core, but also straight out to the edge CPE equipment - every piece of
(non-legacy) CPE that we have has a 1918 address on it that the customer
cannot see.
The only incremental cost is the management of the management network -
almost minimal, as the IP tracking scheme that we have extends nicely into
the /32 range. There is also the required bandwidth, and the need for
additional PVC's to the customer prem (these are 64k UBR PVC's) - again,
minimal when you own the layer 2 network.
Just my 2 cents.
PS - these views are my own and do not necessarily represent those of my
employer.