The only major problem with hosting iurt/jurt/whatever is that it requires either full repositories locally or very good network connection for everything, and - in both cases - tons of disk space..
But yes, I got your point, and I agree with it. Perhaps instead of full 'PPAs' it would be possible to have some sort of iurt-powered public repositories. For example,
http://people..../~user dirs with 'upload' directory there, where someone could put src.rpms and they would be recompiled and stored in
http://people.../~user/{i586,x86_64,arm} when done, with full hdlists.