As a SR3 GM I often felt that karma pool, once it got bigger than about 10 dice, really ended up messing with my games. I felt like the reroll failures use of karma pool, which didn't even burn karma, created situations where player characters could be hit by high-powered rifles but just shrug any ill effects off. For me, I felt it created a feeling of unrealisticness in the game.
However, after seeing some rare camera footage someone uploaded to YouTube, I realized that there are at least some real world situations where people can act like that. The bottom line is unpredictability, I think.
Have a look: http://www.youtube.com/watch?v=xdQldhv_KK4