Bars should really only be used for balance, the vast majority of your roll stiffness should come from the spring rates. Otherwise the bars just reduce overall grip.
Now, I will give you the fact that you will need some bar both front and rear on a nice street car, because enough spring to give the desired roll stiffness would give an unacceptable ride for a lot of people.
In the case we are discussing here, let's start w/ a stock SVT suspension setup for example. As it sits, it has more lift oversteer than most FWD cars already; put a big bar on the back, and you have reduced overall rear grip and increased the lift oversteer enough to be a serious problem.
Now, let's put some aftermarket springs on it; higher rates, relatively balanced front to rear. Well, you have increased your roll stiffness, but relatively evenly, so the need for a much larger rear bar isn't likely here either, only enough to balance the roll stiffness ratio F/R.
Granted, I am WAY over generalizing here, but the rule of thumb still holds that except for very specialized situations (like a dedicated auto-X car that you need to rotate on the head of a pin and don't care that its dangerous to drive on the street) you just plain don't need a giant anti-roll bar. Springs are the big adjustment knob, and bars are the little adjustment knob . . .
