Working in the ProPhoto RGB working space

By the way: If you devide to work in a large Color Space you should work in 16bit to avoid quantization issues down the line when final 8bit output (for web-display or printing for example) is created. 

The gamut of the document color space, and the gamut of your monitor, are two separate considerations. They have nothing particular to do with each other, and they don't need to match.

The purpose of ProPhoto is to give you virtually unlimited headroom in the editing stage. What you want to avoid is clipping colors outside the document color space. If that happens, you can't get them back. You may not even want to preserve these saturated colors in the finished result, but you need to contain them, unclipped, until you're done.

A good example is a raw file out of ACR/Lightroom. Sometimes, you get processing artifacts that result in areas with excessive saturation. To get these areas under control in Photoshop, it's important that they aren't clipped before you get the chance to do anything. Clipped colors generally look very unpleasant, with a dense, opaque appearance lacking in texture and "air".

All that said, there are significant prices to pay for the huge gamut. The one I dislike the most is that shadows get very compressed and delicate adjustments get very difficult. So like Dave I generally prefer Adobe RGB.

ProPhoto is strictly an editing space, not an output space. ProPhoto should never leave your computer. So you might argue that you have to do some gamut remapping sooner or later, and working in ProPhoto just postpones the problem. It is perhaps easier to deal with at an earlier stage. 

The advantage come when you move to another colour space that does contain colours outside of sRGB. For example many printers can print colours outside of the sRGB colour space. Yes, you may* have colours that cannot be displayed on your monitor but if you convert and therefore discard them, you can't get them back even if you get a wider gamut monitor in the future or go to print them on a printer capable of displaying them.
All that said, I tend to work in the middle ground of the AdobeRGB color space which  is close to many wide gamut monitors and RGB printers.

* I say may - because it is the more saturated colours that would be missing so, depending on the subject in the image, you may not be missing as much as you think.
Dave