Graet point.
We do alot of knife stuff in our Jujutsu school,one of our sensei's (who sadly passed on 2 months ago) was very well versed in a few different knife fightin methods, and he felt some of the Prison/Biker stuff was the hardest to defend against.
If your school practices knife defense, where the attacker satnces out and stabs at your gut, leaving the knife out there, yoiu are not training for the reality of how criminal tend to do it, thye recipicate it in and out and usually multiple times. You really have a problem when the alternate hig low, side, up and hit you in differnt spots with their free hand, it's called amping. You have to get off line or jam the knife, or you are in for a hospital stay or much worse.
The more you study kinfe fighting and knifeing, the more you dont want to ever have to face one.
Antoher thing to think about, many prison trained knifers, tie or tape the knife to their hand, so they dont drop it, run their hand up the blade and to make it hard to take away.
Google "Piper system, knife fighting" for some nasty criminal based knife work.