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 The rationale for doing so is outlined here (slightly outdated article, I need to start blogging again!):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