Agree completely with each of Mt. Spok's statements. Read the color management guides at Northlight, and if you want to go further, read Real World Color Management by Bruce Fraser
. It's really the definitive guide for understanding digital color workflow.
Lightroom natively works in Prophoto RGB, and anything given to a print shop or output to a printer should be maintained in that color space (if the lab supports it of course). If you have to downconvert to Adobe RGB or SRGB for a lab or the web then it's the last step that you want to take in your workflow. Personally I find that the Relative Colormetric method with black point compensation works well for most photographic needs.
Personally I shoot with cameras set to Adobe RGB, but would love to see Prophoto RGB available as an option. IIRC at least with the older Firewire 1D cameras you could actually load your own custom color profiles into them. Also, the Adobe DNG converter tool has a camera profiling tool that opens up a lot of possibilities for customizing the way Lightroom handles the color of your images. Keith at Northlight has a good article
on how to use it & why it's useful.
To answer your question as to why
you want to do use the widest color space available for your device, the answer is simply to allow it to record the widest range of colors possible. Why would one want to work in Prophoto RGB when the images coming out of the camera are defined in AdobeRGB? Because once you get the image into Lightroom you start shifting around things like exposure, white balance, and all the other controls that affect color. Manipulating the data in the image can easily push its gamut outside the range of the original color space. When that happens, clipping & loss of detail like the magenta roll of thread in this image
are the result. This article
has another good example of an image with colors that don't clip in Prophoto but do in AdobeRGB & are even worse in SRGB.
Definitely looking forward to trying out LR4's soft proofing like Photoshop has.
It's worth pointing out that having a good monitor and calibration device (& knowing how to use them) are paramount to getting your color workflow close to doing what you want it to do.