For some historical perspective, defsystem-depends-on was added at a time when we wanted a more declarative solution than using load-system in a .asd file, and wasn't initially thought through. It took over a year for it to become an actually useful feature, after realizing the package issue and allowing and recommending the use of symbols in the asdf and/or keyword packages as a workaround.
At the time, the dependency graph of ASDF wasn't understood by anyone, and indeed was broken, and there was no hope of ever turning a load-system into a dependency. Now, the dependency graph is understood and fixed, and there's a way to handle such dependencies right, whether through load-system or through defsystem-depends-on — but it requires a new maintainer to do some non-trivial re-coding of load-asd and/or parse-defsystem. That would mean a major release of ASDF 3.2 and/or ASDF 4 as a result. Basically, you'll need to (a) add new dependency nodes for loading an asd file, (b) properly handle recursive calls to operate from find-system and/or make-plan, and (c) use special variables to track context and maintain dependency graph in presence of such calls.
Will anyone do it? Neither Robert nor I has the will to do it, though I will gladly consult with whoever does it (if he cares from my opinions).
Once this capability is added to ASDF, the rationale for defsystem-depends-on vanishes: just use load-system or load-systems from a .asd file. You may still want to restrict .asd file to a short whitelist of allowed forms to keep it "declarative".
As for what Quicklisp needs, I suppose it is a way to reliably find *all* the transitive dependencies of a system, if possible without loading it, though perhaps after loading its defsystem-depends-on dependencies, which might be required in the general case. Right now, the best you can do is load into an empty image, and inspect all that's loaded afterwards.
—♯ƒ • François-René ÐVB Rideau •Reflection&Cybernethics• http://fare.tunes.org Constitution, article 1: any person who claims that a document called "constitution" possesses any authority whatsoever on other people is an impostor guilty of high treason and liable to immediate capital punishment.