I'm also fin favor of negative pts.
I complete agree with what Free Bagel said:
It better portrays a person's real life skill level. If someone has a fumbling or INT problem, that diminishes their value in real life, and should do the same in fantasy.
In our league it's -2 for a fumble and -1 if it's lost. int's are -2.
In my cousins league it is similar, but they go a step further. It matters the outcome of the turn-over. If a qb throws a hale mary at the end of the half just for the heck of it and it's picked in the endzone, then it's -2. But if the pick is returned for a td then it's -8. More work for their commish, but it take the real life to the different level!! ha ha ha ha!!