Skydog, there is a significant difference between 8 and 16 bit for Curves, levels, and other adjustments in PS when you want to maintain maximun accuracy, maintain maximum number of colors and transitions, noise reduction, avoid banding, and minimizing gaps in histograms resulting from global adjustments (white / black points, curves..). In this regard, PS works in 16 bit mode for all adjustments that matter.
A large number of plugins are designed to reduce detail such as the artisitc effects (Lucis Art, Topaz Simplify) or perform other functions where working in 16 bit provides no advantage and often reduces plugin speed.
For most things you would do in retouching and normal photo editing, 32 bit images result in visibly noticeable difference.
Some people also confuse the bit depth of an image file with their editing software being able to function in a 32 or 64 bit operating system, a very different concept.