I think that this change may be annoying to Unix[1] *users* who start up their lisp from the shell, and who have expectations about how their lisp implementation is going to set the *default-pathname-defaults* from the current working directory.
This change would not break any systems, but it would break some users' conventional usage patterns. I don't see that any benefit is provided that would compensate for that. And I don't see that there is a problem: this only kicks in when people have failed to provide a usable path to their systems. For some people, with some usage patterns and some choice of implementation, the current behavior will provide a desirable outcome. For others, they are no worse off (so ASDF looks someplace stupid where there will be no .asd files, consuming some minimal amount of cycles in an error case).
Furthermore, vendors/implementors for whom *default-pathname-defaults* will not be usefully set, may feel free to define appropriate behaviors.
So I'm not convinced that ASDF is broke, and request that we not fix it.
Best, r
[1] by this I mean the broadest sense of the word, to cover Linux, the BSD family, and Mac OS X.