Denise Cox published 10 good reasons to move your printed newsletter to an online version on her blog. Here they are:

10. Email remains extremely popular.
9. Your publishing costs will be slashed and lead times shortened.
8. Email newsletters excel at customer retention.
7. Email newsletters can be a powerful acquisition tool.
6. You’ll be able to customise the content of each newsletter.
5. Email newsletters are a top ROI tool for your website.
4. Email newsletters are easy to pass along (or print off).
3. You’ll be able to easily and quickly test anything in your newsletter.
2. Your newsletters will now be measurable.

And the #1 reason:

Email allows your newsletters to be Timely, Targeted and Relevant.

Read her full post here.