The point is that a NAS/SAN like OI+napp-it is easy to manage and has nice features. If you are running esxi as the host, with guests living on the virtual SAN storage, it is all in one box and easy to manage. Additionally, you can get very good network performance to the VSAN, since the traffic never leaves the esxi vswitch. As far as drives are concerned, I don't think the intended use is for enterprise level installs where you are going to shell out for 15K SAS drives...
When i say enterprise, i mean anything above hobby/home use. I understand that you get better performance when you host everything on one server and dont need external hardware like a switch, but that also becomes a single point of failure and to me, defeats the purpose of having a SAN when your doing everything local. But again, if just for testing/home use, its fine.