Not just any video, the right video
Video video video. Anyone who has run a campaign with a video component knows that they are more difficult to put together, much more expensive but worth it. From engagement to click through to conversion, video is the way to go. The emergence of pre-roll video ads was simple - the problem is that there wasn't enough inventory so inventory was created in the form of in-banner which would include targeting and other bells and whistles from larger, possibly blind buys. But just because in-banner video exists doesn't mean it's good - or as good as pre-roll:
http://adage.com/article/digitalnext/rats-attic-made-realize-wrong-video-pre-rolls/229934/ , it's the context within which the video ad sits that makes it so successful. In-banner video has all the flare of video ads but it is only one of many vying for attention. Pre-roll on the other hand gets your target when they are captive and receptive: "they've actively selected a video to watch, and will happily tolerate a 15 second ad before watching their video." Of course you're not always going to hit someone that the ad is relevant for but you will increase message recall and brand affinity.