I only pick up those who I feel comfortable I would be able to subdue with the bat I keep in the car if it came down to it.
I also do not pick up anyone hitching for a ride, if they put me and my passengers in danger while soliciting a ride. If they are in the middle of the street, dart out in between cars, blend in with the pitch blackness all around (especially true at night in the mountains) they exhibited enough dangerous behavior for me not to want them in my car.