It depends on the nature of your forum and userbase. Mine is a community oriented High School Sports site and the reason I did it was to make people somewhat accountable for their posts because I have to answer to school administrators, parents, coaches, etc... and they all know me by name and face. Everyone has access to a non-free email account and most people choose to signup with their free account to keep their anonymity. If you were to implement this as I have, the account signups do not really drop off, they just signup with their local email address and that's it. I have only heard a few complaints and I usually work with them to get them signed up!