In my case, I'm perfectly happy with the way Adobe chose to implement this (I haven't actually tried the upgrade yet - I'm just responding to other comments here). In addition to documents that come to me with comments I need to see, I open plenty of random PDFs that have no comments, and in those cases I wouldn't want the pane to take up space.

But I suppose if Adobe wants to make everyone happy, in a future upgrade they could expand the setting to be a three-item radio selection instead of a checkbox:
　Show Comments Pane upon opening document:
　　● Never
　　● If the document contains comments
　　● Always