It's way too easy to create an activity or a workflow that is not linked to a contact. The contact link really should be required, but at the very least you should be able to set your preferences so that either it makes it required for you, or it gives you a warning if you've left it blank. It's incredibly frustrating to realize (assuming you do) that you haven't linked the contact to a workflow or activity, and have to somehow track it down to add it. And then it's even more frustrating as a manager to run a report that has a bunch of activities or workflow tasks that have no linked contact.