Or users creating another account due to lost password (which is easier if you don't have to get another email address).

We use some tools to understand traffic and fortunately most of the traffic is clean. One downside with asking for usernames alone is that people (not too surprisingly), come up with creative but very profane usernames!

Yes. I run a web service that provides a free trial period, and I can't see anyway around it other than forcing a sign up. I'd love to know if there are more friendly ways to do it.