What is the killer part of PPAs? It is pretty easy to setup your own repo on a server using reprepro or aptly. Is it the build-machines-as-a-service aspect? Or the ease of adding a PPA to your system? Or the fact anyone can dump some stuff into a repo and have Launchpad/Canonical/Ubuntu bless them?
I think it's the part of having bleeding edge packages easily linked into your system. Other options might be available but people simply don't know about it. And developers themselves might already provide the PPAs which means there are many PPAs available.
Might be the worst solution but is the most well known.