It doesn't give a realistic picture of autism at all. I work as a guard/assistant/driver at a sheltered housing (my colleagues don't know about my diagnosis) and the inhabitants in this housing with autism are all better functioning than Rain Man, even though they all have varying degrees of mental retardation and none of them are what you'd consider "high-functioning".
The people who live there mainly do their cleaning, food preparing, work in a sheltered workshop and all that themselves, but may need help with tasks such as making a healthy dinner, the can't drive themselves, they have problems applying knowledge, they need help organizing money and so on. They still have a basic vocabulary, they understand that two dollars is twice as much as one dollar, they don't stim much apart from pacing, and if you were to put them on a plane, they probably wouldn't beat themselves in the head while screaming.
The difference between Asperger's and HFA isn't that big, but the difference between autism with retardation and autism without is huge. A problem is that the distinction between AS and HFA is exclusively based on whether they spoke before the age of 3 or not.
Someone who learned to speak at the age of four can be higher functioning than someone who spoke at the age of two. When it comes to Asperger's, who's "mild" and who's not often depends on external factors.
An Asperger's child (even if his AS is "mild") who's taken out of regular school to be homeschooled or sent to a special school in most cases won't be high-functioning. Someone who learns to cope with the world around him and gets the same opportunities of everyone else more often than not will be. A perfect evidence of the latter would be many of the people on this board; many hold degrees, have children, exercise, are in relationships, work and are "almost" normal.