The Artisteer skins we ship with mojoPortal are not really intended for modification, they are finished designs. To modify those designs we make the .artx files available so people can modify them in Artisteer and re-export. Artisteer doesn't provide support for modifying designs after exporting and I don't really offer support for that either, though some people do modify them outside of Artisteer. The skins in the extra skins download are probably a little better for modification (except for the older artisteer skins in the extra skins zip), but some of those designs are a bit out dated as well.
The main reason we include the Artisteer skins in the mojoPortal package is because they are needed by people who must choose one of them as the starting skin when exporting their own design from Artisteer. The choice must be made based on the criteria in the documentation in order to have the correct format in layout.master. Since the goal of Artisteer is to support the people who don't have as much knowledge of html and css we tried to keep it as simple as posisble for those users by including a set of Artisteer skins with each possible layout arrangement in layout.master.
However, once Artisteer 4 ships and we ship our plugin that exports a skin directly we will no longer really need to include a bunch of Artisteer skins in the mojoPortal package since they won't be needed for creating new skins as they are now.
My goal with the HTML 5 Skins project is to get some good designers to contribute some nice skins made from scratch (or from css frameworks of their own choosing) that would be nicer than the current batch of Artisteer skins. If we do get some nice designs we will begin to include those on the demo site and in the mojoPortal package and we will move the older artisteer skins into the extraskins.zip. The main point is that I am not a designer myself and the only way we are going to get some really good skins is to get some real designers involved. I can make a few ok designs in Artisteer but I would much rather have some good designers showcase their own work and share with the community.