Actually, if someone subscribes via email and does not do it through a wordpress.com account, their emails do appear in an area of your dashboard.
My opinion: I would never (really, never) use those emails as an email mailing list, and I would never contact any of the individuals that way without having a very, very good reason to do so. They gave their email addresses in order to subscribe to my blog, not so that I could have their email addresses and send them anything other than the blog updates. I'm completely with raincoaster on this: I'd be really very unhappy if someone were to use my blog subscription to add me to an email mailing list for marketing purposes. You can always edit a contact form to include a field for "add me to your marketing mailing list" and people could sign up and give you their address that way, with explicit permission to email them with other stuff. Otherwise, I'd see it as spam and an unwelcome use of my trusting your blog with my email address, and I'd unsub from the blog pronto.