When you need something more immediate, e-mail can be very effective. E-mail is not an option for every webmaster though. Consider this website. There is no About A Webmaster mailing list and even if there were, it would only capture fragments of our total audience. Other websites do have these types of mailing lists and can use them effectively. Consider forums or online games. These types of websites largely have an audience of return visitors rather than transient traffic. You're also often required to enter an e-mail address to sign up for an account, providing a mailing list for the owners of that website (provided they've clearly indicated how they intend to use your e-mail of course).
The key factor in deciding whether or not e-mail is appropriate for you lies once again in frequency. People generally don't want to get e-mail from you every day or even every week. So you have to balance your need for immediate dissemination of news with the expected frequency of those news events. The last thing you want to do is annoy your community.