It's a forcible interruption of whatever the user is doing, and the user has to interact with it before continuing. As such, it should be used only when there is something that needs attention immediately.

If it's used for collecting user info after they hit a signup button, that's marginally acceptable, but they would still be unable to, for example, scroll back up the page to double-check some info until they finished with it. So I see popups like that as a failure of design; "I didn't know where to put this, so, here. Modal."

What modals are actually used for in the majority of cases I've seen is making modal popups begging first time visitors to register for websites and similar. This task is neither urgent nor necessary (nor, in most cases, desirable) for the users. Forcing them to interact with it before the rest of the page is incredibly irritating, but that will never stop because it's more lucrative.