But then you're relying on every single higher level package owner to pull in patches for all their dependencies and update their binaries in a timely fashion. That works for, say, Debian packages (my distro of choice), but as soon as you move outside of that circle it'll make it harder to guarantee quality.

In contrast, on (for example) Debian, I know that the Debian Security Team is looking after the libraries and that gives me a lot of peace of mind. Instead, I'd be wondering if every single third-party apt repository package owner is doing a professional job. At least if they're shipping binaries that are dynamically linked to Debian packages, I can have confidence that those library issues are being handled. (XML parser update, anybody?)