Why Use ProPhoto RGB Color Space With a Standard Gamut Monitor?

[TomBrooklyn]

1.    Is there any benefit to using the ProPhoto color space when one's monitor is only standard (sRGB) gamut?

1a.   Ditto Adobe RGB with a standard gamut monitor.

- What is the use of retaining more colors if you can't see them?

2.   Are there any possible DISadvantages to using a wider colorspace than you can see?  

3.   If printing, how can you softproof your photos and visualize the printer output, if the file contains colors you can't see on your monitor?

[Bob_Hallam]

Many Inkjet Printer / Paper combinations can get close to the ProPhoto gamut, Monitors can not. So color adjustments must be done on-screen relative to an actual calibrated printer using that specific printer profile in PS will not hurt but will also be subject to the monitor's gamut clipping.  So the proper workflow is to Calibrate the printer,  then use that profile in PS to convert to from ProPhoto RGB.  Color correct from the print, not the display.  That way you will have a successful color-managed workflow.