In the end, it's all about time. People are rational creatures (despite rumours) and quickly decide whether they are willing to spend time to do the email activation thingie, and so they bail. Someone should do a survey of conversion rates vs time it takes to sign up somewhere, i bet there will be a strong correlation.

Anecdotally, i ve observed the opposite too. I don't require email to sign up, but there is an email field further down in the form, and yet 95% people DO enter an email that looks valid, and not just garbage. That said, only 5% clicked on the email verification link , presumably because they don't have to