[ferretchucker]

It's an interesting question. Whilst I've been debating the status of Godzilla (2014) as a horror film or not, in fact I mostly agree with David Bordwell on the matter; genre is not the most useful form of classification.

Our current notion of genre derives from both literature, and from a time when films were far less saturated. It exists to help prospective consumers by setting up a series of expectations - "oh yeah, I tend to enjoy films which are sold as "horror" so I'll probably like this one." But now every broad genre is comprised of subgenres, films tend to include significant aspects of other genres in them now etc.

Take Seth MacFarlane's most recent outing - A Million Ways to Die in the West. The title suggests two things - most obviously western, but also elements of "horror." In actual fact, it's much more comedy than either Western or horror, but it features significant gore and death that could align it with horror from an aesthetic standpoint. The fact is, we rely far too heavily on these outdated terms.
Film audiences these days tend to have a pretty wide range of reference materials to draw from so categories can afford to be more specific, in my opinion.

Another example - Tarantino. So he's made gangster films, a western, a war film etc. But more than anything, telling someone that something is a "Tarantino Film" is the most useful way to sell a film to someone.