You must meet modal windows all the time when you're designing for a web site or a tablet App, and you must have multiple ways to close a modal window.
Obviously you can give the user an 'x' button or a cancel button, you can also allow the user to close the window by clicking outside, or you can have them both.
But we have different use cases with the modal views.
It's very easy to get the window closed by clicking outside, because you don't need to click or tap the close button accurately, and it’s also intuitive to click the main page under the modal thing to get back. But the suitable condition is when the content is read only or the user won't make big effort on it. Once the users will input many text within the modal window or make big effort on it, they don't want it be closed by a careless touch.
If a modal window will let users to make big effort on it, we'd better let the part outside the window un-clickable and un-touchable.