Today, the Gmail plugin and the Outlook plugin both log emails and SFDC activity to the first recipient in the To: field of an email. While this is handy, the reality is most sales exchanges occur between more then just one sender and one recipient. Oftentimes you are working with a team of people, all of whom need to be involved in a conversation. If you really want to track engagement with a customer, you need to know everyone who is being talked to.
Other automation programs log to all recipients, so it seems like a feature Marketo needs to adopt to address actual business practices. Without this feature, it is impossible to report on true activity.