Personally, I think of modals as a way to slap the viewer in the face with an important action or information.
Dialogs, photo/video viewers, notifications, help tips, contact forms, etc. all are common use cases for modals and they all are purposed for a single task. Usually solving something that doesn't make sense to require a page load, or something important enough that needs to be noticed before a page load happens.
When the user clicks a sign-in link, for example, all they care about at that point getting to the place where they can sign-in or register for your site/service. They do not care at that moment about any other "important" elements on the page. A center modal login box that grays out the rest of the screen puts the form directly in front of the user, without the need for a page refresh or having a login form on every page. But, you need to make it easy and obvious for the user to "escape" or exit the modal logically.
tl;dr - Modals are understood/designed to be simple, don't add complexity to them.
Great article here from Smashing Magazine on modals - lays out use cases and examples as well as all the different types of modals and what works for what.
Also, although it's a bit older, this thread touches on similar issues you discussed in your question.