Pros & Cons

Pros

Increases your influence and power when you can call upon a massive Social Action Team (your subscribers) to take action.
Low cost-per-action.
Social Action Team size and growth can be an impressive credential for grant proposals.
Excellent for volunteer retention and up-selling.
Highly scalable: comparatively little extra effort is involved in mailing to 2.5 million subscribers versus 250,000 subscribers versus 25,000 subscribers.
Extends relationships with website visitors from momentary to longer-term, providing additional opportunities to convince kindred spirits to take action.
Deepens your understanding of each responding member's motivations and behaviors.
Content developed for email alerts and newsletters can be leveraged for use on your website, and vice versa.

Limited to "preaching to the choir" rather than outreach to the undecided.
Requires sophisticated tracking and database programming to do well.
Typically, click‑thru rates plummet as mailing list size grows. (Deliverability problems get progressively more complex and frustrating as your list size increases.)
Creating advocacy email that's worth reading can be time consuming for your internal experts. Publication deadlines may be missed.
Even conscientiously‑managed bulk email will generate complaints, which are time consuming, irritating to handle, and may fuss Board Members.
You'll be competing for attention, deliverability, and reputation with spam email for Viagra, penny stock tips, and penis enlargement. That causes many legitimate email professionals to mumble when asked at parties what they do for a living.