In my experience low-quality PRs aren't that common, but I do agree dealing with them is annoying. You can't just tell people to go away because they did spend their spare time on it. On the other hand it's also garbage. Sometimes it's garbage by people who really ought to know better. IMHO low-quality issues are the bigger problem by the way, a problem that existed well before GitHub.

But I just don't see how GitHub or a PR-style workflow relates. Like I said in my own reply: I think it's just because you'll receive less contributions overall. That's a completely fair and reasonable trade-off to make, as long as you realise that is the trade-off you're making.