I think that your argument suffers from mixing together the concepts of "version control" and "versioned software." Version control is indeed the concern of version control systems such as git and subversion. But versioned software is not, as witness the use of version management in software installation systems such as RPM and apt.
Indeed, if everything is managed in a single, or a set of interconnected VCSes, version control is less important, as you point out. However, when one is not using VCS, e.g., when using quicklisp, or assembling software oneself from multiple locations, versioning (not version control) is important to efficiently assemble a coherent set of subsystems.
Even when using VCS it can be helpful. I work with people who concurrently work on multiple different lisp-based projects. Those projects use different mixes of libraries, and at times even different versions of the same library in different projects. Sometimes, inadvertently, ASDF settings can bleed through from one project to another. In such cases -- event when VCSes are pervasively used -- it can be quite helpful to have version metadata.
Best, r