People who want to run ZFS for storage usage. ZFS is still better than Linux alternatives like LVM, BTRFS etc. To be picky, there is not an exact counterpart to ZFS under Linux.
And with clones etc. you can set up 1 master KVM image and then clone it 10 or 100 times, saving a lot of disk space in the process (ZFS clones only use the amount of space that is different from the original).
People who have a lot of Solaris experience or have to maintain a lot of Solaris systems already. Also people who are using Solaris zones.
People who are building backend systems that require or can use any/all of the above. Right now, no one cares what your Web SaaS service runs on, as long as it stays up and doesn't lose their data.
Solaris still has a better VM subsystem, IMHO, than Linux does, and performs very well under heavy loads that include disk activity.
Regarding zfs: the zfs on linux project (http://zfsonlinux.org/) is actually fairly mature and has decent performance ( though there are still some outstanding issues with using zvols as xen filesystems, and also if using a xen paravirt kernel ).
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.
People who want to run ZFS for storage usage. ZFS is still better than Linux alternatives like LVM, BTRFS etc. To be picky, there is not an exact counterpart to ZFS under Linux.
And with clones etc. you can set up 1 master KVM image and then clone it 10 or 100 times, saving a lot of disk space in the process (ZFS clones only use the amount of space that is different from the original).
People who have a lot of Solaris experience or have to maintain a lot of Solaris systems already. Also people who are using Solaris zones.
People who are building backend systems that require or can use any/all of the above. Right now, no one cares what your Web SaaS service runs on, as long as it stays up and doesn't lose their data.
Solaris still has a better VM subsystem, IMHO, than Linux does, and performs very well under heavy loads that include disk activity.