Previous to Jira 5, we used a "Email This" plugin and have stopped using it in Jira 5 given the new Share feature. But the Share feature should have an option to include the contents of the share email in the issue activity trail.

Example, I want to see that I sent a link of the issue to a coworker on May 22, 2012 at 9:05am with the message "I need you to take a look at this issue. How should we fix it?". This is essential to keeping track of the lifecycle of the issue.