Okay, First off, I'm an OpenIndiana user, on a laptop, and we use OpenIndiana on a few servers, and it is great for this role.
I have to say however that the kind of proselytising, and almost idolatry, which you are exhibiting towards Illumos is offputting even for me.
ZFS is the most awsome filesystem out there, but please keep in mind that Linux has ZFS, if you add it as an additional, and *BSD has it native.
Yes, Illumos/Solaris networking with crossbow is better, but Linux is _good_enough_(tm) for almost all uses out there.
RBAC is great, if you trust people to do specific roles on specific servers, but learning how it works is probably on a par with modifying your sudoers file for specific tasks in Linux. And if you don't trust people, you're just going to end up with a limited number of people who can assume root, where you're back to where you started.
and I don't know what you mean by "Illumos uses the Distribution model" ... almost all operating systems do something like this ... in fact you can make your own Illumos Distro (if you're willing to hit that learning curve) in the same way that you can build your own Linux, I don't see how this is a differentiator.
You didn't mention that Illumos (at least OI, and smartOS) has KVM support native to the system, which means that they're catching up to Linux for virtualisation.
Reasons not to use Illumos:
USB support is sh*t ... connect more than ~ 6 devices and you'll soon find that those hard disks aren't available, very not good if you're using ZFS on raided external disks.
USB3 support is non-existent.
Fuse support is mostly broken.
FAT disk support is limited to almost Dial-up speed, NTFS support (because of Fuse) works some of the time and kernel-panics the rest ...
Hardware support is limited. Where hardware is supported it works very well, but if it isn't supported then you may as well go fish.
Although Illumos development is supported financially by companies, OpenIndiana is not, and is run by Individuals in their own time, so development of applications (Desktop and Server side) can be slow to very slow.