I've got the 16-35 and have had a few situations where VR is critical.
The primary situation is for landscape images where a tripod does not provide stability or can't be used. For me, shooting a landscape image from a boat in the Okefenokee is a typical situation. The only options are from a canoe or a flat bottom boat. Sunset images are from a boat - usually with other people. It is impossible to be completely still, and a wider aperture is not a solution. VR makes a big difference.
I do a lot of stream photography, and tripod legs in moving water don't provide complete stability. There is always a bit of vibration from the fast moving water. VR helps these images to be sharp even at shutter speeds slow enough to blur the water. I'm not sure of the limit of VR, but at the slow end I'm using it at 2 second exposures. It's also stream photography with the Vari-N-Duo where I need the filter threads on the 16-35.
Another situation is a bit of a compromise. Sometimes you cannot get the elevation needed for an image. I've actually used a tripod held at arms length above my head to get a better perspective. VR allows this extreme handholding situation to produce usable images. The wide angle provides some forgiveness in framing.
The final situation is in places that prohibit tripods. I have some interior images from churches in Italy that would not be possible without VR.
All these situations probably account for less than 5% of my images so VR is not critical overall. But it sure is nice for certain situations.