If the queen chooses poorly, another queen can come along and take it and bestow it on someone worthy. So, I do think they try to find good recruits to everyone's satisfaction.
small thing
i disagree.
The books show us that there have been a few hundred years of bad recruits and Mab is far too intelligent for that to be something she didn't want.
My thoughts are that the stuff about her own knight being a threat to her is true. Knights focused on drugs or rape wouldn't be any threat to her, and until now she didn't need a powerful knight or a loyal winter lady.
Mab Knows that Ragnarök is coming and over the last decade or so it seems quite clear to me that shes been preparing for the war to end all wars.
side theory now it seems to me that mab has delt with both winter ladys, her handmaiden and winter knight and destroyed the red court id say Titania needs to be delt with next. shes obviously too unstable and depressed to lead the summer fay into war, she would be a massive liability.
also with Harry's mysterious and powerful lineage and his starborness. There are all kinds of interesting plot lines Jim could come up with for Mab wanting a child with Dresden. Also we didn't see Mab for at least 9 months after she knighted harry and when we did she was visibly weakened and fay have showed a strong desire to have a child with Dresden before. Id say he most likely has a changeling child out there with Mab