Author David Meerman Scott recently wrote a terrific blog post about free ebooks and how distribution is affected when registration is required. It's easy to think that "free is free" and requiring registration shouldn't have much of an impact, but check out the stats in David's post.
OK, the numbers presented there aren't exactly ironclad or statistically irrefutable but still, it's an interesting study. I tend to agree with David that there's a valuable trade-off in getting your content into more people's hands for possible Twittering, blogging, etc. In many cases these benefits can greatly outweigh the value of capturing email addresses for future use.