The plain text version of your message will be read by constituents checking their email on their Blackberries or by those who do not accept HTML-based emails. This is likely to be a minority of your subscribers, but it's a good idea to fill in the plain text section nevertheless.
Links in the plain text version
Because plain text does not support active, clickable hyperlinks, you'll want to include the full address for any site that you'd like to link. You might want to change your language from "click here..." to "go to..."
The [link] field
If you insert [link] into the plain text, it will be replaced with the URL readers can visit to view your email in a web browser. You can use this feature by typing "To view this email in your browser, go to [link]" at the top or bottom of the plain text version.