There is always a need for rotate functionality. It's quite weird that you have flip and mirror and don't have rotate. It's quite useful when you deal with scans made with scanlite. Reproducible: Always
Apparently the Gwenview developers agree, which is why there are Rotate buttons in the default toolbar, and why mousing over an image in Browse mode displays Rotate buttons. So it looks like this is working properly. Let me know if I haven't understood your request correctly.
Or maybe you're talking about the Image Operations panel, where the rotation actions are indeed missing. In that case, this is a duplicate of https://bugs.kde.org/show_bug.cgi?id=354149 *** This bug has been marked as a duplicate of bug 354149 ***