Files in this directory represent development toward a "stable" version. These "fragile" releases are meant to give people a chance to test the intermediate versions out and provide feedback - perhaps bugs, perhaps discussion about what seems to be a poor direction to take Logisim. You can also use them to get access to the "latest and greatest" features of Logisim.
However, I wouldn't recommend working much with these development releases, because there is no commitment to supporting these versions in the long run. If you build something with a fragile release, it is possible (though unlikely) that you'll end up building something that can't be loaded properly in a later release.