At first I was very confused because Midnite and I can see your summary page when we go to your profile and we can also see our own summary pages when we go to our own profiles. But then it struck me that sometimes we can see things that regular members can't. I couldn't imagine why SMF would have created a new permission to allow or disallow members from seeing their own summary pages - but I was wrong because sure enough, when I just went to check the permissions for regular members, there is indeed a new permission in SMF 2.0 exactly like that.
However, I've now set it so that everyone should be able to see their summary pages.
And I'm sorry that checking permissions didn't occur to me earlier. Though who knows why a permission like that even needs setting in the first place?