We are also struggling with how to manage subscriptions/unsubscribes. I would like to see this on the on the development road map sooner rather than later, because as you point out there is a real risk of not honoring the preferences people select because there is no fall back. I think Marketo needs to allow for true unsubscribes to happen by category, rather than the all-or-nothing unsubscribe approach available now. This will make it easier to both ensure people get only the category of communications they want, and to see in reports the true unsubscribe rate (since deselecting a subscription preference now does not show up as and unsubscribe).
Is there somewhere else where this topic is being discussed with regard to Marketo's road map, or is there a better way for me to log this as an enhancement request?