I get the thought that pinned tabs are done with the thought of the URL (domain) not changing. So, when a new URL (domain) is identified a new tab is opened with the new URL.
However, I like to use a pinned tab for music. I have several music sites bookmarked in a folder (Pandora, DigitallyImported [and its sister sites], YouTube, for example). When I want to change which site (via bookmark) I'm listening to, instead of updating the pinned tab, a new tab is popped out and I have to pin the new one while deleting the previous one. Not user friendly.
My idea is to have a setting for any given pinned tab to remain in the tab when site changes or to popout to a new tab. Another option might be, if the new site is bookmarked, then remain in the current pinned tab.