In fact, as evidenced by the developer guidelines, the pivot is not even intended to be used as a navigational control the way a side panel is. In some cases it was just misused as such, probably because MS does not provide developers with any standard approach to navigation at all (except the hub, which is appropriate only for media intensive apps).
This is so spot on. There is no traditional "tab control" in WP. The Pivot, which looks an awful lot like a tab control, was specifically NOT supposed to be used in that way. In the way that Microsoft envisioned and documented it, each panel on a pivot was to show the same data types and formats, but with different filters applied to each. The Email app is a follower of that model: each panel is a list of mail with a filter applied (none, unread, flagged, or urgent).
As a5cent said, lacking anything else, developers "misused" (well, I prefer "extended") the Pivot control to create tabs. I do it in my own apps because there is no provided and instantly recognizable alternative.
I just took a quick look for pivots in my biggest app. The first three links from the initial pano page go to three Pivot pages, each using a different "tab model". The All Courses pivot had panels for this semester and next, so it follows the MS-preferred pivot usage of each panel being a filter (i.e., different semester filters of the full course list). The Schedule pivot has panels for Daily View, Weekly View, and Finals View -- this isn't quite what MS had in mind, particularly because the latter two are grids while the first one is a list. The Info page totally ignored MS with "tabs" for Release Notes, Tips, an Feedback.
I won't even make an argument that I followed the guidelines for pivots. I also won't apologize for it because 1) the "rule" is too subtle, 2) users aren't even aware of the rule and actually do think that pivots are "Metro tabs", and 3) there's no alternative offered. In my view, as long as the panel appear related to their parent page, anything is OK. So, if I'm on a page titled "All Courses," I should not see tabs for Spring Semester, Sports Scores, and Settings. (One could argue that those would be a fine set of tabs for a page called "Things that Begin with S," but this isn't a Jeopardy category. :wink: