I think we can say the opposite, too. Using 3rd party unsupported/unchecked repositories is just that - unsupported and unchecked. Users should be aware of that. IMHO The non-presence of an autobuilder in extras would not lower any barriers, it would just make it more of a dumpster (as then it's every developer for himself) - and then there is no difference if it's just one or a hundred repositories as the contents would get quite messed up over time anyway.

Disclaimer: I'm using launchpad ppa's, pbuilder and similar stuff for quite some time so I might be too easy on the autobuilder approach