Ok good. Then how about having ASDF loudly announce that it is registering the default directory (e.g. ~/cl/), as well as providing the hint about using a .conf file with the right syntax with :tree, if the user wants to exclude certain subdirectories.
The goal here is to make the addition of the default registry directory as painless and surprise-free as possible for anyone who already happens to have stuff floating around in the new default directory.
FTR, when i wrote my own version of :tree i also added a loud output that listed everything that got added to the registry, because previously i've wasted way too much time with loading the wrong systems, and with debugging build differences between dev environments and production deployments.