One thing to remember with working with different colour spaces is to use the one that best fits your image. Whilst ProPhoto has a wide gamut the data is still represented by an 8 or 16bit image, this means you lose tonality for the sake of the extra colour space. If your image doesn't use the extra colours in the larger space you have effectively reduced the number of colours you have available so you're worse off than working in sRGB, esp. if your device driver only support 8bit data.
To illustrate although it's not a true example:
Let's say you work in a colour space that has 512 shades of grey, the 8bit image is now trying to represent 512 shades of grey using 256 values. If your image has black to mid-grey gradients you will likely see banding because you've lost the tonality
