When you serve out an entire system for a group of diverse users with lots of installable programs, it's important to ensure that parts will work together. Especially in Unix where namespace clashes in /bin, /lib and /etc may cause serious problem for system administrators and users. Thus, every stable release has to ensure that none of provided software shall do nothing nasty.
But with bleeding edge, you lose the control over this. It may be worth it for some users, and not for some others. That's a trade-off, and I myself am at the maintained-package-repo+use-only-stable-releases side; thus I use FreeBSD. Debian and Ubuntu mainly provide stable server OSs, so they're on my side too. If you want bleeding edge software, you should migrate to an OS that provides bleeding edge software, tho should know that it's called bleeding edge is called bleeding for a reason. An OS that has to release a version that'll be stable for the coming five years simply cannot provide bleeding edge packages.
If what you have is fine and works, why want new software? And if you want more frequent updates, you can (a) use a faster-moving branch, which BTW Debian and Ubuntu have; or (b) switch to an OS that provides you with more recent software, e.g. Arch Linux, Manjaro, Gentoo (? not that sure about this last one). Declaring sth. stable takes time, especially if you also incorporate new stuff into it. Even after five years of development, Debian and Ubuntu release patches, because of errata in their releases.
But if you want secure, stable OS that wont drop the eggs, and also rolling release, well, nobody can do anything about it.
That's OK. Nobody has an objection that you do it that way. It's just that there are acceptable flaws and shortcomings for some use cases, and some OSs care more about those.
With functional package management (as in GNU Guix or Nix) you no longer have namespace clashes and packages can be added without too much concern for unrelated packages.
I find functional package management to be a vast improvement over the packaging situation --- unlike incarnations of the latest bundling/appification trend such as Snappy or Docker.
I didn't say I want an entirely bleeding edge system. I said a bleeding edge ubuntu, by which I mean keeping up to date on the non-LTS track (I updated my original post to say that, because I can see where the confusion lies here).
Out of the 2500 or so packages installed on the computer I'm typing this on, I want about 10 of them to be bleeding edge. Maybe 20.
So isn't having a separate channel for those 20 that change and update often but thus more prone to risk than the other 2480 packages that you probably would like to work and not bother you a good thing for your productivity and security? Now I last used ubuntu 2-3 years ago and IDK a lot about PPAs, but, what are the shortcomings, exactly? Clashing dependencies (i.e. sth. in your ppa wants libbob 2.0, but ubuntu has 1.8)? The packages you want don't have PPAs?
PPAs would work if they would be widely adopted by all the upstream software whose latest versions people desire AND the PPA infrastructure is adopted by all major Linux distributions so that developers don't need to package in N different formats. But as it stands, leaving it up to distribution maintainers means programs tend to lag several versions behind, while for faster updating each distribution has its own incompatible way, which means developers often give up and don't package at all and leave it to the maintainers, or just select one method like PPA or AUR.
As far as I can see, Snaps/Flatpaks aim to change this, but at the expense of introducing much less manual oversight and inefficiencies in terms of storage, at the very least.
I'm really not sure what you think I'm arguing here. The situation I have with Ubuntu+ppas is satisfactory to me, and it is also anecdotal evidence that "leave it to the maintainers" (asserted by the OP) is not a complete solution to real problems (my assertion).
But with bleeding edge, you lose the control over this. It may be worth it for some users, and not for some others. That's a trade-off, and I myself am at the maintained-package-repo+use-only-stable-releases side; thus I use FreeBSD. Debian and Ubuntu mainly provide stable server OSs, so they're on my side too. If you want bleeding edge software, you should migrate to an OS that provides bleeding edge software, tho should know that it's called bleeding edge is called bleeding for a reason. An OS that has to release a version that'll be stable for the coming five years simply cannot provide bleeding edge packages.