What is the "main Debian system" ? stable? testing? unstable?
well...the debian's PPA haven't realized yet so we can only hope they take account that matter when designing it. It will really nice if the future debian PPA could work at least for both stable and testing. it's true that sid is constantly moving but the build library due to their development period will freeze for every 6 months.
Example, is it possible to build a recent version of plasma desktop in debian stable right now, after less than 1 month from stable's release? If ever such PPAs would exist, how can it supply plasma without introducing newer system libraries?
I'm not quite sure but it should be possible. as long as the API didn't drastically changed the package maintainer could try to port and build the packages using older build libraries. the code might modified and due to the API stuff some of feature might be removed but surely we can do it. the case is similar with linuxmint with their cinnamon. the gtk-tools used to build cinnamon is different but they succesfully build same version cinnamon for both LMDE and the main version. in addition, qt is less disruptive than gtk so we have higher chances to do it for plasma.
The idea for having a repository is so that other people's programs would be tested and working with the entire system, and not break it or require something that's not in the repos. And too I think it helps to eliminate malware risk (not that Linux is even prone to it as much as other OSs).
I believe it's the opposite. if we talk about official repository then what you said might true but if we count 3rd party repositories it's the complete opposites.
1. it's not tested. the repo owner build the packages with his system and might not works with other system.
2. it's not safe. basicly you didn't know what or how the packages compiled. the owner repo might unnoticedly insert some malicious code.
not to forget that if the owner repo forget to update his packages you might ended up with serious problems. since PPA is based on source code everything is transparent for both fellow developer and the user. thus, it's lot safer compared to 3rd party repositories.
as I've said earlier PPA-like infrastructure have becomes common feature on modern distros. ubuntu have PPA, arch have AUR, while fedora and opensuse have OBS. fedora even move futher with copr, koji, and other suttfs. in summary, all major distro seems already have it. debian is conservative so it's quite understandable that it's the last distro who implement it. however, just like the case with systemd they can't avoid it forever.