You've explained who Solaris is for, but if you want Solaris why would you want to run a fork of a fork of Solaris? It's still not clear what value this adds relative to Illumos.
For one, KVM virtualization does not run on Illumos, nor Solaris.
Second, Oracle is encumbering Solaris with a not-nice license. I haven't followed the latest developments because I no longer care, since I will never use a non-free OS.
Whether you meant to ask, "Why Illumos over Solaris" or "Why SmartOS or OmniOS over Illumos?" I think I have answered your question :-)
illumos is free and (most importantly) open source. Running a closed OS and binaries on a production server isn't the most foolish idea I've heard, but it's in the running.
