On a crop sensor camera a 50mm lens gives an equivalent focal length of about 75mm for Nikon and 80mm for Canon. This gives a greater distance between camera and subject, and crops out part of the image from the lens, and therefore two things happen:
1.) Distortion from being too close to your subject with a 50mm lens is eliminated because you have to stand further back to get the same framing. This pushes it a little closer to being a standard "portrait" lens, especially on a Canon.
2.) The weakest part of the cheaper 50mm lenses are the edges where fringing, aberrations etc. occur. But because the sensor is smaller these weaker areas are effectively cropped out all together and we are left with only the best part of the image! Oh yeah!
At the ridiculously low price of the "standard" 50mm f1.8 lens this means that it makes perfect sense to invest a small bit of cash to get near "pro" results! To get the same quality from a zoom you will be spending a LOT more money, and it will be larger and heavier to boot.
More "pros" than will care to admit it carry the nifty fifty in their bags, just ask them for a peek in their bags, it will be there somewhere. Ask Chase Jarvis, he has one in his bag too.
And if you are an "oldie" photographer and like the 50mm focal length because that was your favourite back when you still used a film camera (remember film?) then consider the Nikkor AF-S 35mm 1:1.8G DX for a crop sensor Nikon cameras as it gets closer to what is considered a "normal" focal length which in this case equates to 52mm. I also hear good things about the Sigma 30mm f1.4 DC HSM for most brands of crop sensor cameras. These are both great for half body, full length or environmental portraits and also for small group shots, giving superb performance for a low cost, although I from what I hear the Sigma might not be as cheap as the Nikkor.
So if you have examples of portraits with the "nifty fifty" then please post a link to them below for the benefit of us all!