In lecture 6 of "How to start a startup", entitled "Growth", Alex Schultz touches on this point specifically in a hypothetical concern people might have of being too spammy. His point was that one should worry about the marginal user who doesn't get notifications, not about the power users who are grown ups and can set filters. Granted, SO is no longer a "startup" but growth is still relevant to it nonetheless and it is interesting to note that the question of emails is far from being over and done with.