In my [limited] experience PR have always been orders of magnitude better [for me] than the average issue. Maybe it's because they focus on improving the project and not user's problems. It is true however that some times they were unsolicited and/or unwanted, however they were still a lot easier to process and review for me as a maintainer.

Of course if you remove issues and reroute people to PR the quality of them will lower (though not so low as Issues) while you will miss important things, but still seems like the net result is a easier to review _issues_ which might be what saturated maintainers want.

Edit: I totally agree with the last paragraph (and parts of others of course).