As far as I know, though KMers have been trying to measure the benefits of KM, there is still a large amount of ambiguity and vagueness involved in the exercise. Is anybody questioning the necessity? Maybe. Maybe not.
The benefits of the existence of the HR function isn't measured...is it? The ROI as in the work that managers do (they don't work but get things done) isn't measured...is it? They why is it that KM needs an ROI. It should be fine to measure the # documents, # hits on the intranet, # queries posted to experts, # ideas from communities etc. But is it possible to get an ROI? Even if we can, can it really be attributed to KM? We might be cheating ourselves by saying that the ROI or cost/effort saving due to KM is $ 12345... Do we want this despite the fact that we are aware of the ambiguity and difficulty in attribution? Is it worthwhile to pursue this effort?
The process is as important if not more important than the end. If one were to listen to music for the joy it gives one, it is ridiculous to measure the benefit or say that it gave us 4 1/2 minutes of joy. It is ridiculous to bring up one's kid with the ROI in mind. The kid might take care of its parents in their old-age but that's not the reason why parents bring up their kids. They enjoy the process, see the need and responsibility that lies on their shoulders, understand the implications, and love it per se.
Einstein said it well: "All the counts cannot be counted and all that is counted does not count!"