The 10 bit color extension would be useful even without extended gamut because even the traditional gamut would have more color resolution - i.e. more in-between shades for existing the color spaces. This makes the use of external processors for shading, grayscale and color tweaking more usable without compromising performance.
Using external processors with 8 bit color inputs limits what you can do without causing crushing, which is why it is usually preferable to achieve calibration using the device alone. With 10 bit color (and higher) I can actually see using external processors to finish tweaking the calibration in cases where projector's limited built in calibration tools leave off with a less than perfect result.
I do think that for existing sources no use should be made of the extended color gamut - it would just lead to oversaturated and inaccurate colors - although there may be people who enjoy this. Perhaps video games or some proprietary new titles in the future will support "deep color" but there's no mention of it in the BR and HD-DVD specs.
Finally, if projectors are tweaked to provide maximum contrast at an extended color gamut, when you constrain the gamut for existing color spaces and source materials, you would be losing brightness and most likely contrast.