And using this service of course partially defeats the point of using Debian stable at all, at least for the packages that you install from backports.

I can imagine it being more useful for things that are easily fixable/less critical.

I was running a testing desktop, and X updated in a way that broke the proprietary nVidia drivers (nVidia's fault -- not Debian's.) That was quite annoying to suddenly have to fix. Those types of issues (though rare, even in testing) would be nice to avoid.