I have commented once or twice on the Reclaiming the Mind blogs. I have never signed up for a mailing list with them. Yet, in one day, last week I received nine emails about supporting their ministry, or some newsletter, or blah blah blah.
First, I don't think it is a good idea to harvest emails in your comments to send mass emails. That's for sending out notes that someone else commented on the post (a feature I still need to add, though that feature should be opt in as well) or for the writer of the post to use to communicate with the person individually if he/she so chooses. Second, it is certainly not appropriate to just sign people up and send them emails to ask for money or tell them about the cruise you are planning that you think is so groovy. For those who would want the newsletter, you've made them happy. For all of those who wouldn't, you've just become a spammer.
I am considering creating a way to have posts and such emailed to people for my blog. Some people don't like feed readers. But I'm not going to harvest emails from my comments. And if you are going to make something similar for yourself, you shouldn't either!