It depends on the output. Many printers can print outside Adobe RGB, so for printing it makes a lot of sense staying within Prophoto RGB.
For web display sRGB is the best choice, but that may change with 4K which has a much wider colour space.
In my case it is easy, as I normally work in Lightroom which uses a colour space with Prophoto RGB primaries, do everything in Prophoto RGB and export as needed.
But, this discussion started about accuracy of colour rendition. What I have seen is that exporting in Adobe RGB reduces colour accuracy in both LR 5.3 and Capture 1.
prophotoRGB is very much an intermediate color space; not an output one, kinda like the prores format is for video editing. Finish all your edits in prophotoRGB, use the convert to profile dialog box in Photoshop, chose "Perceptual" and output to whatever your preferred output colorspace is.
This has worked really well for me so far.