He might have other workload he wants to run on that same server. There's nothing wrong with virtualizing the core (or anything else for that matter) if you have the reason and know what you're doing. I'm running 5 other VM's in addition to a virtual LMCE core, which lets me experiment with LMCE without disrupting the "production" environment. I'll probably stand up anther Virtual core as a dev environment to work on some stuff, and connect it to the virtual MD I sometimes play with.
On a six core AMD 2.8GHz system with 8GB RAM, I'm running the Core, a Windows Home Server 2011 instance (iTunes server for all the iDevices), a Astaro VPN end-point server, my mail/calendar server (Zarafa), MrHouse home automation, and a Ubuntu Desktop environment as a virtual hosted desktop. During peaks, my load is about half what the system is capable of; I'm I/O bound more than CPU bound due to SATA disks. When SSD's drop in price, I'm going to pick up a bigger one for the VM disk storage (I have the various databases on LVM's from a 40G SSD to handle the IOPS requirements, and to lessen the load/latency from the disks).
As far as networking goes, I have two NIC's in the system. Eth0 is br_ext, a bridged network device connected to my home router which gives outside access. Eth1 is br_int, another bridge, but for the internal LMCE managed network. The core owns that network, so any physical device connected to the switch on that NIC is seen by the core, as is any virtual Server connected to the br_int. The virtual core's eth0 is connected to br_ext for Internet access, and for access from the existing production environment. It's eth1 is connected to br_int, and it provides dhcp and all related services to the "internal" network, as per LMCE's architecture. So, I can run both environments in parallel, without affecting the wife and kids.
So, the takeaway is, if you have a reason to virtualize it, go for it. If you don't have a use-case figured out, then you might not want to, because it can be more work. With great flexibility comes a lot of (your) overhead.
Hope that helps!