I use it on my MBP for one reason: less strain on my clicking finger, which in the past has led to RSI. Using my old Thinkpads with the TrackPoint and clicking was much more stressful on my hands than my current setup.

I don't have any trouble with errant clicks, but I may have altered my typing style. Right now my hands are both entirely off the keyboard (elbows resting on armrests on chair). I also understand there are some improvements in Mac OS X over other operating systems with respect to errant clicking.