I disagree. There are enough applications out there that *use* KDE but are not *part of* KDE (I worked on one, but they do exist). Some of the things offered in kdelibs make things painless (KPlugin, KParts, etc.) and if using them means you're tied to KDE releases, I would think that it's a hard decision between dropping those technologies or saddling up to KDE's release schedule.
It also means that you have to download *all* of KDE together. Of course, KDE could do like they do on Windows and have a package manager of sorts, but I don't know that that's the path that upstreams should be falling back on because that'd only bring us back to the same situation we have today (one supported version, keep updating or you get nothing new).
I don't think it'd be too onerous to get kdelibs and the core GNOME libraries pinned with a stable API and ABI (KDE has a policy, but it does get broken at times).
Of course, then there are projects like Boost which would need every version packaged in the base distro (expecting every project that uses Boost to deal with building and linking it on their own is asking a lot IMO).