I don't disagree with your reasoning at all, but I do keep my main tree private because there is a lot of speculation I don't want propagated – I’m still very new at all this. I did however make a stripped down version with relatively accurate names, dates and places that is attached to my DNA for precisely the reasons you mentioned.
I think the problem is we assume people take the DNA test to connect with cousins or help fill out their trees and I don’t think that's necessarily the case. You can tell by the topics on this board that many people are focused on the ethnic breakdowns and either don't want to (or have the time) to go through other than very close matches or ones with hints. Think about how this is advertised – it says send us your DNA and we'll tell you where you came from. There is nothing about you then spending massive amounts of time following up, and I'd be willing to be many people do not. As mentioned on another thread there are also a lot of "undernourished" public trees with less than 10 people probably for the same reason, and those don't help us either. And yet another issue you see are people who haven't even bothered to attach a tree to their results even if they have one, leaving us to guess who they are in their tree or trees.
And finally, we're assuming people read these message boards, and most do not. That means the bulk of the people this conversation is aimed at never see it and will continue as they are. It's like there are two main groups here - people already doing genealogy who are adding DNA as a new tool, and people who are not (or are casually) and just want to see what the DNA says. I'm not trying to be pessimistic about this issue, but I don't think the second group is going to change their habits no matter how many times we send them message. I think the only way this issue will get resolved is by changes from ancestry on how they deal with public and private trees.