This discussion has naturally followed the path of highlighting the differing pilot preferences with respect to flying or riding to the ground following an "emergency". Those who would choose to have a parachute available are entitled to their opinion, though I do not share it.
As I think back over 35 years and 6400 hours of flying, I do not every recall a situation when I would have wanted to have a parachute available. When I flew jumpers, I was required to wear one, but only because of the risk of my being flung out of the aircraft's open door. I was happy to trust the seatbelt, they insisted on more protection.
A number of valid points have been made about the landing gear and general arrangement of the airframe being different in twins so as to loose some of the occupant protection characteristics common to BRS equipped types. Add to that the cost (both initial, and on going maintenance) of the system, and carrying that weight, and most pilots would shy away. Yes, a few conditions mentioned, like a forced landing into rugged terrain make the parachute attractive, but a pilot can choose to not fly there too!
Happily, pilots are not demanding en-mass, the provision of parachutes, so obviously there is a very low perceived risk of needing it, and I support that view...