Great thread...there have been some good posts! Here's my $0.02.
Why Train with a knife? Several reasons: 1. Because in the hands of someone who knows how to use one (or in the hands of a complete idiot), a knife is a devastating weapon. In my personal opinion, at close quarters (striking range or trapping range) the knife is probably the most dangerous weapon you can encounter. 2. Knives are one of the few weapons that can be carried almost anywhere. Granted, there are some places you can't carry one (airports, gov. buildings, etc.) and different states have different laws regarding type, action, and blade length, but for the most part you can find one (or two or three

) that you can carry. And like nonono90 said, they are just an extra tool in your S.D. toolbox.
On the subject of knife defenses I have four rules: 1. don't make someone mad enough to pull a knife on you. 2. If they do, run like hell. 3. If you can't run, shoot them ("never bring a knife to a gun fight"). 4. If you can't shoot them pull your own knife and hopefully kill them before they kill you. (and if you don't even have a knife, R.I.P.

)
I agree except that I would also say that in a "real life situation" against an
untrained knife-fighter, disarms by a highly trained unarmed person would be difficult. Even if you succeed in disarming them, you are probably (if not definately) going to get cut in the process which brings us back to my first 4 rules for defense against an armed attacker (see above).
To be clear, I'm not dismissing knife training (I love it) or saying that defensive tecniques will never work ('cause it has happened), I'm just making the point that a knife is a very formidable weapon regardless of whether your attacker is highly trained or not.