In my case, I don't see any reason - especially now that I am using DP's add-on that shows more ads to guests. I've never attempted to force people to sign up because most users can get their info from the existing million+ posts. I see almost no advantage to new members......others than a few relatively serious ones who want to ask or answer questions not already taken care of. It's an interesting topic - that is, the value of guests as opposed to visitors. I seem to have maybe 6 to 8X as many guests as visitors at any given time.