"When the image color space is ProPhoto RGB it is much larger than the printer color space. The profile must compress the image color space into the much smaller printer color space and this may cause poor quality prints unless you have expert editing skills. It is much easier to achieve excellent results when the image color space is approximately the same size as the printer color."
Well, the above quote is simply wrong...nothing about printing is "EASY" and trying to make people think a smaller working space will make anything easier is naive at best and disingenuous at worst.
The "size" of the working space is not really an active factor when determining the "skill level" required on the part of the user. Yes, ProPhoto RGB is a really, really big space. I use it to make sure colors that my camera can capture and colors that my printer can print don't get clipped by a smaller working space.
As gvdavewh says, there's a lot of color that _CAN_ be printed on today's high-end pigment printers that can't be contained in Adobe RGB. So, working in Adobe RGB not only clips colors your camera is capturing but that can be printed.
As long as you have an accurate display profile and an accurate printer profile, there's no particular GREATER difficulty using ProPhoto RGB than any other working space and several benefits.
But the choice of working space is really something people need to determine themselves...listening to ANYBODY else is really avoiding the issue by avoiding doing the testing and making a determination, based on the results, for oneself...if you want to make maximum use of the camera colors and the printer colors, as I do, then there's really only one choice of working space. If those issues aren't important–and only YOU can determine that–then use the space that works the best for you.