Let all those "bad PRs" with useful features and fixes accumulate at your own peril. You might wake up one day and find you're not upstream anymore because someone else has merged them all into a fork. I've seen it happen.
You seem to assume that in all cases such situation would be a problem.
In fact it not always is a problem. For some I would love if someone else would maintain it, for some fork is friendly and has a bit different purpose and so on.
no, I am not obligated to merge badly written PRs introducing bugs just because I had no time to implement the feature myself