Interesting... does this imply that when editing images for web publishing, it is in practice better to use Adobe RGB than ProPhoto as working space after initial RAW conversion in order to avoid artefacts? If I'm not mistaken, when targeting high-end printers ProPhoto would be better since printers can have wider gamut range than monitors?
The answer to that is to use the most appropriate color space for your needs.
ProPhoto has exactly one advantage: the huge gamut. But as with everything else in life, it comes at a price. The price is the compression of middle values, which is where you are most of the time. Subtle adjustments are difficult, because each numerical step covers a much larger distance in actual color.
I use ProPhoto when I need to. But I prefer Adobe RGB. It's much easier and more pleasant to work with.
Also keep in mind that editing in ProPhoto just postpones a problem you have to deal with sooner or later: gamut clipping when converting for some output or other. You don't want gamut clipping, or not a lot of it at any rate. It usually looks pretty bad. And if you simply convert a ProPhoto file to sRGB or an average print profile, you're probably going to get a lot of clipping. It's simply easier to deal with at an earlier stage.
In other words, working in ProPhoto can increase the risk of a poor output result.
Yes, I hear the argument all the time that some high end printers can print some colors outside Adobe RGB. While technically true, my response to that is - so what. Good color isn't about maximum saturation, it's about color relationships. Those printable non-Adobe RGB colors are a small range of already extremely saturated colors, and in a very limited hue range. To me, that's an insignificant consideration weighed against the other inconveniences and shortcomings of ProPhoto.
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