If we can skirt the discussion about bunnies and trees for a second (keep this up and you'll be conducting an on-line seminar on tying bowline knots.....or worse).
Stegner, as Reccesoldier and Bomb Chucker said, the MPs still have free will. They do toe the party line but that is an act of volition. They do it because they believe it is the right thing to do, or they fear their fates too much or their deserts are not small. In a distorted "democracy" they sell their votes for their future reward as a minister or a critic and the increased perqs.
In the case of Dion's Liberals there is precious little in the way of deserts that he can offer as a reward. His coat-tails are as long as the average mess-jacket's.
In fact, far from having coat-tails he is an anchor. Montreal and Toronto Liberals have a better than even chance of getting elected regardless of who the leader is. Folks like Bob Rae, Ignatieff, Findlay and Coderre will likely keep their seats even if the Liberals get knocked back to 20 to 50 seats.
As to primaries ...... just more bloody complications and machinations. Keep in mind the Yanks use the Primary system to vote for their King-pro-tem or Governor-General. They are filling an entirely different billet.
Aside from wanting the Senate gingered up to become a house of provinces with single term senators I don't think there is much wrong with our system. Nothing that regularly changing the government won't cure.
Und Herre Gott Bombchucker, mussen wir wieder Deutsch gelernen? >