One of the best things about SoftSynths is that they use midi.
Midi in the box is a lot more powerful than midi outside the box.
Every time you run a softsynth you're running a whole 'nother
port with another 16 midi channels .... and no midi cables.
And so on ad infinitum ( or until your cpu chokes).
Another plus that comes to mind is the huge ammount of memory
a PC has to offer .... many sample libraries today are way too big
for a standalone hardware based synth/sampler...
Sure the old analogs are hard to emulate exactly but most of the
analog emulations are so close it only really matters to real hard
core afficionados ... most of us haven't even heard the originals
to be able to compare ... so the emulations sound fine.
There are more advantages ... but that will do for now .....