I gave this a great deal of thought when I was buying my car. In the end, I ended up getting a car without one. Why? I liked everything else about that car, and I live in san jose and these cars grip well (much better than a mustang I owned years ago
). However, as CJ said, it is an option, so its good to be with it than without.
To answer your specific q about PSM being a necessary option or not, again quoting CJ, it'd depend on the weather and the kind of driving you do.