I think Dean was plenty devoted to John (fair warning: I have lots of John opinions) so I wouldn't really compare directly Mary to John, or for that matter Mary to Sam. In general I do think it is a positive and necessary step for Sam and Dean to develop (nonromantic) external relationships with other people who can/will survive their shitshow lives.

But I do get that Dean would worry about taking her into dangerous situations, even if I don't agree that that's his decision to make (either with Sam or Mary)! Haha, I love what you said about how he mothers people, up to and including his own mother.

I think part of my frustration was very much 'why are they even back at the bunker in the first place? Did Dean decide to drive back there specifically to try to dump Mary instead of following leads to where Sam is being held prisoner? WTF?' But that's, I'm sure, giving the writing more credit than it deserves. I just have to keep reminding myself not to overthink bucklemming episodes, because it is like trying to run through a dark narrow tunnel with a ceiling you keep bumping your head on. :/