It's much more realistic if you leave it on. On anything stickier than gravel, ABS has advantages that are impossible for any driver to produce without it. Namely, the ability to relieve each corner independantly.
So just about any modern sports car, and these days just about any car with over 130hp, has abs.
And there is no reason to "turn it off." BMW, well known for their stand on 100% defeatable driving aids, does not offer the ability to disable abs.
The origional Dodge Viper has no ABS throughout its entire production and in every edition. Look up some old comparison articles and notice how the Viper is ranked at the bottom in braking performance. Thats why the new one has ABS.
I was once enjoying a midnight drive in a Viper ACR and it was not a gentle drive. At one point -the highlight of my night- the Viper's dim... innadequate headlights revealed to me gravel spilled on the road. I had a turn coming and I had planned on turning ...fast... so I was approaching at an appropriate speed for my plan. Slowing that freakshow down on a narrow, wavy, road with ice-cube sized gravel scattered about was a revelation to me. Because I've done the exact same thing countles times but never once in a car without ABS.
The way modern sports cars are designed, I'd consider abs a basic component of the braking system.
So. Yeah. Don't turn it off.