Hanzou
Grandmaster
I have a different opinion on this. If you have some actual skill in grappling... as in, you (as an individual) have developed some reliable, applied expertise in grappling... you can do some experiments to layer in additional elements. This idea that BJJ guys can't train knife defense is stupid, and yet we get this straw man all the time. "BJJ/MMA/Wrestlers say all the time that they are impervious to being stabbed because their abs are so rock hard." Okay, maybe not exactly that, but just as ridiculous.
@Tony Dismukes has shared some really interesting experiments that he's done over the years, and there is real value in that. It's useful because it's layered onto actual skill. He has the expertise to evaluate his skills. Or said the other way, a person who lacks grappling expertise is in no position to evaluate knife defense.
I can see the logic behind that, considering that an experienced grappler would have knowledge of controlling the body.
I do think that people whip out the knife thing as a gotcha against the grapplers. It's like "Haha but what if I have a knife? Take that Jiujitsu!" Seemingly ignoring that they just admitted that they need a concealed deadly weapon to have a chance against your martial art.
That's like one of the highest compliments a MA could receive.