2-3 stops is enormous. Speed of the lens is partially where the additional cost comes from, and professionals are willing to pay for that extra performance - especially sports photographers.

It's been a long time since I paid any attention to pro photographic equipment so I don't know if fast lenses are important with modern sensors performing so well in low light. But back in the analog/early digital days 2-3 stops was a huge advantage.

Also something to consider, the pro lenses also focus faster and more accurately. I was never sure if this was down to the software in the lens, which is proprietary and why 3rd party lenses are never as good as manufacturer. I could easily see Canon or Nikon tweaking the firmware on prosumer lenses to focus a fraction slower.