If you have a beast of a computer (both CPU and RAM), by all means, do what works for you.
Sampler instruments (like Kontact or Halion) can get very RAM intensive and using a single instance might save on this.
Another nice thing about using a single VSTi for multiple things is drums, where it’s convenient to have one MIDI track for the drums, while still having multiple outputs to mix them differently (and using effects etc).
A friend of mine bought a new super PC for his musical work and even with multiple heavy VSTis (EastWest), the system barely felt it.