No, not really. A large gamut per se helps nothing. Mor important is, how the colors are treated, are out-of-gamut (OOG) colors clipped or mapped into the working gamut. And if you do not want to do any post processing, you don't need the 16bit color depth. If you can do all corrections within your Raw conversion software, it would be ideal to convert it directly to the output profile. That's unfortunately not possible with Photoshop, so there you have to convert the image into an intermediate color space first. Due to the architecture of the RGB profiles in PS, there is no use in ProPhotoRGB as an intermediate color space, if you want to convert it to another RGB space afterwords. It only makes sense, if you convert directly from ProPhotoRGB to you printer profile and this profile (and your image!) contains a considerable amount of colors outside AdobeRGB. If this is not the case, you loose nothing converting directly to AdobeRGB. If you have to do some major postprocessing, you'll might want to go 16bit to avoid posterization. If you do only the conversion from AdobeRGB to your output profile, I am not sure if 16 bit has any significant advantages. Using ProPhotoRGB makes 16bit a must.
I strongly disagree with most of this. There are several advantages to using ProPhoto as your editing space. First, when editing an image, there is no advantage to converting from RAW directly to a specific output device profile, and many disadvantages. Most photographers print their images on more than one printer, whether personally owned, the minilab around the corner, or an online service. You want to have at least one master version of your image that is not compromised in any way by output device gamut limitations, and then create a device-specific version for a particular press or printer only when necessary. Converting RAW to ProPhoto first, then editing for a specific device allows much greater control over how the gamut compression/reduction is done. Why cripple your master image to accommodate the current limits of rapidly-advancing printer technology?
If your histogram is perfect, when ProPhotoRGB is the output profile, there might be some clipping switching to AdobeRGB. You have to counterbalance this with the exposure settings.
This is really bad advice. The exposure slider is at best a very crude way to control color gamut. It only helps control gamut in the highlights, and has no effect on saturated colors in shadows, like green leaves or grass in the shadows of a landscape image. These colors are often more prone to fall out-of-gamut than bright colors, unless you're shooting autumn leaves or car shows or flowers or suchlike. What you're advocating will result in unnecessarily darkening the image when saturated colors are present, which means more and unnecessary adjustments later in Photoshop.
That's, what you have to care for, and that's where large gamuts are really difficult to handle. A perceptive conversion from ProPhotoRGB to sRGB is not possible, so in order to prevent clipping of OOG colors, you have to carefully desaturate the colors using the soft proof function.
That's how you should always adjust gamut to fit a specific output device; adjusting individual color ranges with the hue/saturation control, preferably with a saturation and/or luminance mask to focus the effect on the color ranges that need to be shoehorned without affecting the rest of the image any more than absolutely necessary. Wide-gamut editing spaces are only "difficult" if you don't know how to use them effectively.
The final conversion from 16bit to 8bit is not a problem, since you need the 16bit only as a headroom for corrections. When those are done, there's no need for 16bit anymore. As long as your output device works in 8bit, as well. There seem to be printers with 12bit output, regarding Michaels report on the new Canon printer. This of course, would be a definite reason to keep the image in 16bit.
Keep your image 16-bit anyway, unless sending it to a client that can't handle 16-bit images. Even if you are printing to an 8-bit device, you'll still get better results printing a 16-bit file. When converting from 16-bit ProPhoto RGB to 8-bit printer data during the printing process, all 256 levels of the color channel data going to the printer can be used; the color profile conversion is perfomed first, then the data is rounded to the nearest 8-bit value and sent to the printer. But if the source file is 8-bit, then you may be only using 100-200 of the possible 8-bit values in a color channel (think of the "toothcomb" histogram you get when doing a curve adjustment in 8-bit mode in Photoshop) and you'll have more posterization and banding in your print as a result.
As long as your output profile is considerably smaller than your "large color space", there's only very little advantage, but big problems. There is absolutely no use in a large color space, if there are no colors using it. If the colors of your image are within AdobeRGB, a gamut like ProPhotoRGB is a waste.
This is really irrelevant when working with 16-bit files. ProPhoto files are no larger than Adobe RGB, and neither are susceptible to banding/posterization in 16-bit mode.
With my limited knowlidge and experience, I find it more usefull, to compress the colors into AdobeRGB, than using ProPhoto and struggeling with clipped colors later. Since we in Germany have a pretty well organized CM with well supported ISO CMYK profiles (not the crap PS profile named "ISO Coated Fogra 27" but the real ISO stuff), I find it a bit annoying not to be able to choose an ECI-RGB or any printer profile directly in ACR - that would solve the whole large gamut 16bit discussion.
This advice is based on the limitatations of your knowledge and experience, which is why you're "struggeling with clipped colors later". Using RAW converter exposure settings and output color space choice to fix out-of-gamut colors is crude and imprecise, and leaves one with problems that must be solved later. Also, be aware that given the differences in shape between printer color spaces and editing color spaces, you need ProPhoto to use 100% of the printer gamut anyway. Many printers can print some colors that fall outside of Adobe RGB, so you're possibly cheating yourself out of some printer gamut if you use anything less than ProPhoto anyhow.
And if you start using a different printer with a wider gamut, then you have to completely reprocess your image from RAW conversion on to take advantage of it. If you have a master copy of the image in ProPhoto RGB, you only need to perform a hue/saturation adjustment on the master, convert to output profile, and save as a new copy (and even this will be unnecessary on many cases) and you will not have to redo noise reduction, sharpening, curves, compositing, cloning, and other adjustments and edits to the image. This is particularly useful if the image is a stitched panorama, an image shot with mixed lighting requiring multiple RAW conversions with different white balances to be blended together, a multi-shot high-dynamic-range blend, or has some other time-consuming processing that must be done.
Get in the habit of editing a 16-bit ProPhoto master image that is not compromised by output device limitations, and making output-device-specific variations of the master when necessary. It will save you time and hassle in the long run as printer technology improves.