In other words, in synths, the slideshow has never been connected to the highlights. They are two separate ways to explore the synth, just as Overhead View and 2D View are different ways to explore it.
When panoramas launched, highlights got a little upgrade as in panoramas they only made sense if you could select portions of the panorama. This carried back into synths as well, allowing us to select only a portion of a photo, rather than the entire picture.
What about slideshows for panoramas, though? Panoramas aren't like synths where you get to move all around an environment. If you've shot a panorama to be free of stitching errors, you haven't moved the camera lens' position at all, so the slideshow can't be a 'tour' like it would be in a synth. So, in panoramas' case, the Photosynth team made the slideshow follow the highlights since there's nowhere to go in a pano.
In photosynths, tho, the slideshow's purpose is just to tour through every photo in the synth.