Not required, just makes the car handle better during cornering. Most cars prior to 1973 or so didn't have one, only performance models got them. Most all modern cars have them, and police cars have had them since the 1960's, so what does that tell you..... nice to have but not manditory if your not chasing handling improvements.
This is assuming you are calling a rear anti-roll bar a "sway bar".
Or are you calling the factory panhard (side-to-side axle locater) bar a "sway bar"?? What is the suspension you are using??