While editing images (color adjustments, tone adjustments, etc.), you need a big enough working space so that you don't accidentally clip colors by exceeding the working space's gamut. ProPhoto has a big gamut and it's very unlikely you'd go out its gamut.
The problem with using the output space (e.g., the color space determined by a specific printer profile) as your primary working space is that the output space is typically much more limited in gamut. So you have less room in which to work, so to speak. Also, what if you want to print the same image on different papers or different printers? Then you have to convert from one to the other. Not necessarily a bad thing, but suppose the target device has a bigger gamut. Well, since all of your colors are already clipped to the first device's smaller gamut, you won't realize the benefits of the target device's bigger gamut.
As an analogy, suppose you moved into a tiny studio apartment with 1 room. You can't fit all your belongings in that 1 room, so you throw some out. Then a month later, you move into a 2-room apartment. Now you have more space in which to place your remaining belongings, but you can't get back the ones you already threw out earlier.
Eric
