The main advantages I can see of softsynths are:
1) They are cheap in comparison
2) They don't weigh anything
3) You can always have multiple instances
I'm not even going to say that they are reliable, because my computers will crash more frequently than my Moog will blow up on me. The Moog might go down in 10 years of use, maybe once? Any computer will go down and cause issues MUCH more frequently. Likely monthly at least. Ok, maybe they stay in tune more? But that is not the only measure of reliability. You can fix tuning, you can't fix your own softsynth when the code is broken.
1) Better sound
2) Better looking and more impressive to clients
3) Better workflow and interface
4) It's actually an instrument. I've had to explain to some of my non-musician friends that a Moog isn't just another keyboard, but an instrument unto itself that requires learning to play it. All pianists aren't great Moogers
5) As long as you maintain it in some very basic ways, it will last forever. How many 5 1/4" floppy drives do you still have around your studio? How many 8" floppy drives? Software obseletes itself and becomes unrunnable, and unsupportable after a while. Hardware doesn't. A Moog from 1973 will still run fine with a little TLC. Try using some software from 1973. Forget it. Your copy of FM7 will NOT run in 2026. And perhaps it won't have been ported to the new system then either! TurboSynth doesn't run on OS X for example.
So for me, I can't understand why people want to use softsynths, asides from being cheap, and having no space. I am cheap and have no space, and still prefer hardware.