Elliot,
Sorry for the complicated question. It’s probably easier to explain with a screenshot 🙂
As you can see, the default value for the select box is “Select.” So if I create a new custom field, then come back to edit this post, I can tell I haven’t set that custom field before. Whereas with your plugin the default is “True” and I have no way of knowing if I set it to “True” or not. Does that make sense?