Many of the monitors in general use are not able to display even the full Adobe RGB gamut let alone the Kodak ProPhoto(=ROMM) gamut or the LAB-space that extends, in principle, to every hue that the human eye can distinguish (but in reality only 10% or so more colors than the ProPhoto space).
So ADOBE RGB is probably sufficient for current RAW processors. There is still an argument for using ProPhoto to represent hues that can be reproduced in some media but that lie outside the ADOBE RGB gamut. That is why I save all my images out of PL5 using a ROMM-RGB profile in the export dialog.
Inkjet printers are, of course, still more limited in the gamut they can reproduce but it’s typically not orthogonal with any of the foregoing gamuts: the hard question is how to fill their potential gamut with the best translation from the editing workspace (a discussion for elsewhere).
I think the main advantage of working in a LAB workspace (which I would REALLY like DXO to provide, but I’m not holding my breath) is that it makes it possible to work with development tools in a luminance-only (‘L’) channel mode. This can be invaluable for e.g. curves adjustments. I REALLY would like to see this in PhotoLab and I note it would FIT THE NAME of the product. A secondary advantage is that it makes color-contrast a target/tool of development rather than – as it is now – a hard-to-predict effect of general contrast tools (e.g. “Selective Contrast” in PL) that affect RGB color contrasts indirectly in a much less transparent or accurate manner.
In the absence of a LAB workspace, however, I’d also be very glad to have a Luminance-only channel for the Tone Curves tool.
P