This was my answer:
A knowledge sharing initiative is a "response" to a problem. It is not something you do for its own sake.The bottom line here is that to my mind "you don't do KM, you respond to business problems and opportunities with appropriate KM tools and techniques".
What business problem are you trying to solve? Do you want marketing to work better with R&D to develop new products? Do you want salespeople to understand more about your products to increase cross-selling? Do you want programmers working on a software product to communicate more to avoid design oversights and bugs?
Start with the problem(s). And then ask how can "knowledge sharing" help in responding to them? If you are responding to a business problem then you have your justification!
Oh by the way improved "knowledge sharing" is probably only part of the response to any business problem.
Also note I say "response" and not "solution" - in complex systems - there are no "solutions".