why VST's in the first place, ......why doesn't anyone like the sounds already in a keyboard.....who's judging anyway....
just sayin? after almost 50 yrs on stage no one has ever said anything about the sounds I use....  
 
 I don't get it? whats all the fuss? 
Offcourse we like the sound in the keyboards of today, my T5 is fracking brilliant..
But somehow VSTs are so much more flexible and have so many more options..  SA and SA2 sounds and ensemble voices is something that comes from the VST world, and i can name atleast 10 more top features only available in VSTs these days..
I will give you some examples...  
-There is pianoteq, which obviously is the best most naturall piano sound created, you will not hear the difference between a top grand of 200k and pianoteq.
-There is huge and broad orchestra sounds, which you can play live and sound like a real orchestra, ensemble on stroids with 40 to 60 instruments
-I have a set of synths from arturia that just sound like the real instuments, ARP2600, minimoog, jp80, CS80, and more..
- omnisphere that lets you design any sound you want just from the ground
- and all those sounds are edditable in real time by just twisting some knobs
- i have Korg VSTs that sound like the real instruments
- drums with roundrobbin that just sound like they are plyed live with all imperfections.
In the end VSTs are like having dozens of different instruments inside a single box..
If you want to sound just like the orriginal artists then an arranger does the job very well, butIf you want to create new sounds and your own compositions or just play with sounds, then VSTs really add a lot...
I think for  the needs that most people have on this forum, they will never need anything more then a T5...  But then others just love moving forward and just playing with all those musicall toys.. For me its fun, and i can toy around hours with a new sound, tweaking it till it sounds just perfect and then finding a use for them...