...as blueark states, there are pros to using rgb colors, but there are also cons, color management is a big area and is only any good with accurate and high quality profiles. In some cases simply converting to cmyk from rgb can have colors go whacky, and depending on how color critical a job is, you have to then adjust the image in a cmyk space, like blues turning purple etc...
...just depends on what your dealing with, but i certainly wouldn't be creating text or vector graphics in RGB colors, if i wanted text to be 100K ink then i would make it so rather than picking a screen black RGB mix...
...bitmap images are different beast, and for sure RGB has advantages here early in the workflow...
...essentially from a prepress point of view, two images the same with different source RGB profiles converting to the same cmyk destination means you get a different color image. This can be a problem in a "re-print" scenario unless you have strict colour policies in prepress for all users...
...without a closed loop you have clients wandering why their reprints are coming out different, potentially a nasty/expensive situation...
...and on the other hand, no two print providers print the same in reality, using different systems, inks, papers etc...
andrew