For a conventional CL implementation I don't see the value of the monolithic fasls, although I suppose there would be some speed-up in loading. But I would guess that if that's what you care about, you would be better off using an image to load.
We use the monolithic fasls a lot, as part of our standard Gendl and GDL build process. The rationale for doing so is outlined here (slightly outdated article, I need to start blogging again!):
http://gendl.blogspot.com/2013/03/saving-images-with-asdf3.html