What's the value to advertisers in online video ad networks selling in-banner inventory?  Can't advertisers buy banners from display platforms (sites, networks, exchanges) and then just run video through a rich media vendor?

Regardless if this is effective for the advertiser or not (hint, it probably isn't), the value lies in campaign scale, pricing, and ease of implementation. For scale, there is far more in-banner inventory our there than video player inventory so coupling the two strategies together allows a vendor to increase the reach of the advertisers campaign. Since the ad network probably buys the banner inventory in bulk, the might be able to realize some economies of scale which allows them to lower the CPM and capture higher volumes of business. The network may also even get the inventory on exchanges and just run the in-banner video so there is pricing efficiency there, but that also leads to my next point which is about ease of implementation Also, going to one vendor and having them deal with everything soup-to-nuts is a lot easier for a buyer to manager than have to plan, execute, and optimize across several vendors.

This is what we do and I can answer "yes" to your question. Build one ad that works on both desktop and mobile and run it anywhere. No need to be locked into a network.