Just a few years ago there weren't much Engines you could license, but today there are a lot and they aren't that expensive and some are easy extentable.
The downside of licensing would be probably that they can't provide tools for the community to build mods/levels and so on. On the other side, if they build the engine themselves, that will bind quite some resources.
So what do you think, should they build inhouse tools to create the game or license a engine like Unity3d/UE3/Cryengine and so on?