Ethics is one thing, but forcing a candidate to produce a competitor's offer seems intrusive.

I had a (great) boss in my late 20s who was overjoyed to have me share my offer with him so he could go to bat with the corporate overlords (in another state) and get all the local devs a raise based on market conditions.

I was more than happy to share, but we also already had a relationship where I knew he wasn't out to screw me.

I have never shared one companies offer with another, nor would I. Of course if I company was not willing to negotiate I would immediately walk. I have just learned a lot about the people who work there.